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While I'm not sure on the rules for all copetitions and all nations, for teams entering the FA cup, the pitch dimensions cannot be altered from the ones submited on the entry form at the start of the competition.
I would assume the rules would be simialar for many (but probably not all) european countries.
I've just looked at the champions league ruels as well. Fo this competition, pitches must be 68 x 105 metres unless an exception has been granted by UEFA administration. I'm not sure if gaining an exception is just a formality or not, but requests are only accepted before the start of the season, so in this case also, you would not be able to change mid season.
I might do a bit more research to find out the rules in some other competitions, but then again, I might not.
In the premier league handbook, it states that pitch dimentions cannot be changed mid season without prior written consent of the board.
It also gives some more info on the rules for UEFA competitions, basically the only time clubs are allowed not to use 68 x 105 sized pitches are when the stadium construction does not allow for this.
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