You do not need to switch an existing service unless you want to keep your old number.
FreedomPop lets you chose a new number from a list of available numbers when you open an account. You need to do this even if you decide to keep your old number. Once you have the service activated, you can request to move your number from your previous carrier and drop the temp number you started with. Or you can just keep going with the new number you selected.
To answer your other question about how difficult it is to cancel a previous contract, that all depends on the rules your previous carrier has. I cannot answer that. I can only tell you what to expect.
If you are still under contract, they will either not release your number or they may charge a termination fee. You need to contact your carrier for the actual details.