Rating and opening

Are player ratings based on the openings they play?

For instance,

A 1900 player who has only ever played d4 ... If they play e4, will they retain their 1900 rating performance?

So, perhaps a 1900 d4 player is (in this example) only a 1700 e4 player? Does this make sense or just nonsense?

Actually it makes perfect sense.

If a player never played 1.e4 in their life time than yeah they would be a patzer in playing 1.e4

It doesn't mean they will play extremely terrible though. For example if they are 1900 and are a 1.d4 player they have been able to master other area's in their game along the way. Middle game, Endgame, Tactic's.

Which can be used in some 1.e4 lines. However, opening/line wise they may not be as great. They may even fall into opening traps that they are not unaware of.

Usually when people switch openings like that from like 1.d4 to 1.e4 you see their ranking drop alot. Its becuase they have not mastered the lines that follow from 1.e4 vs 1.d4 ones.

However, with continues practice they will start to get back to their old ranking. An or if they switch back to 1.d4 will get back to their ranking.

A few things to consider here - first, we'll state the obvious: It's hard to play anything you haven't at least looked at....some of the really uncommon openings like the grob can simply be felt out positionally from either side, but those things are not played because they are not very good. However, if you've made it to 1900, chances are you're not pig ignorant regarding 1.e4 structures - you've seen games in books and lectures featuring the advanced caro kan, the French, the Sicilian, etc. And you know your own opening choices against e4. It's true that the d4 player making a switch is going to bump into something they haven't seen (usually some crazy gambit), fall into a trap, and simply lose, but, again, you dont hit 1900 without having a real understanding of how chess works at the mechanical level. 

However, individual preparation style is a huge consideration - some people are real bookworms (I count myself in this group, unfortunately), and they don't want to play anything they haven't repeated ad nauseum at an analysis board. At the high level, Levon Aronian is certainly one of these: He plays his openings very predictably, and he's noticeably uncomfortable leaving familiar territory. Alexander Morozevich is the opposite: He has a few patterns, but, really, just re-prepares a whole new repertoire for every tournament (which is INCREDIBLE!) - Moro switches constantly, it's impossible to know what he's doing from either side.....and he's been in the top 20 since the Kasparov era! Which would tend to indicate that rating and repertoire switching don't have an exact correlation.

The next thing to look at is the fact that prepared openings - no matter how deeply booked - cannot last more than 20 moves or so at any level. Eventually, you're going to have to come up with a plan and execute it without the help of highlighters and stockfish. Here's where we look at Mamedyarev - a middlegame genius of the first order. This guy plays 'weird' stuff all the time - he doesn't care what is evaluated as 'fashionable' and 'better for white' this week....Shakh just wants a position where he can try to kill you, and he's racked up over 2700 rating points doing just that. A player like this asks "How do you hold up once the book runs out?" 

Finally, we have to consider chess holistically - as a game with three distinct, interconnected phases. The aforementioned Mamedyarev will never be world champion - why? I would put him up against Bobby Fischer in a tactical middlegame, no problem - he might be *better* than Fischer in these situations........but Fischer - the quintessential holistic chessplayer - has a chance to outdo Mamedyarev both in opening preparation AND in endgames, and, in a match, the former World Champion would prove himself superior to Mamedyarev based on this skill-edge he'd have in 2/3s of the game. 

Peter Svidler said in an interview that the difference between a truly elite player and a lesser player (even a grandmaster) is that elite players will be tested on EVERY element of the game, and, if any part of it is lacking, they will not advance beyond a certain 'glass ceiling'. 

In other words: Sure, preparation is important, and your repertoire will be a big factor in success or failure, but the real path to greatness is getting to a place where you can study, play, and experience the game holistically. 

No it's logical. If you consistently play the same openings, your pieces will reach similar squares and thus you will have recurring tactical and strategic possibilities. Over time you will become more atuned to these possibilities both from actively recalling them (maybe you have played them before to effect, or saw them in a GM game) and through intuitive feel for the position.

When I decide to venture into new opening lines that differ a lot from the ones I generally play I feel a fairly drastic decrease in playing strength.

The famous American coach Dan Heisman once said in one of his Novice Nooks that "give a 1900 a position from an opening he's never seen before and he can still "moreorless" play like a 1900, and give a 1200 his favorite opening and he still plays like a 1200, so instead of memorizing opening moves, work on stuff like tactics, etc."

I do believe that if you play an opening you have never played before, your playing level should decrease somewhat just because you are unfamiliar with the subleties of the position and typical tactics, but not necessarily to a patzer level.  Unless you got to 1900 purely from memorizing absolutely everything with no understanding.

i was 1700-1850 in blitz and then changed to 1. d4 and now i am 1850 - 2000 blitz (1900+ in normal) ofcourse there might be other things as well that steps in