Well I guess they can tryout at the regular tryout time with everyone else then. I am just naturally wary of the player who wants to jump teams in the middle of the season. There have been a couple of times when my DDs have felt like the teams they were on were not the right fit after a few tournaments. In those cases they played the season out with their current team and then went somewhere else. I know there can be cases of legitimate harmful environments on teams but from my experience when a girl wants to jump ship it is usually about playing time or the parents want to get her on a "better" team.
The stay and finish your commitment is all fine and good providing a valuable life lesson. However, once you get to 16U-18U and into the recruiting process staying with a team that is a poor fit is not an option. If you are not getting the exposure and/or the coach is not holding up their end, which is quite common you need to find a new home ASAP.