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. |