The shards work by limit. So you could end up joining a mature shard with higher level and developed teams.
When I joined my shard, it had a maxed out Phoenix squad for ages, though thinking on it that may have been a cheater as the top 5 is populated by lots of Palp leads with G11 only maximum.
The shards themselves are 20000 player limits. Hence why you see people gaming the system by timing their entries (level 28) so they can get in at the beginning of a new shard.
However, once you are in a shard that's your shard for the rest of your time in the game. No moving around.