It would depend on the availability of your new practice
If you're no longer registered at the practice, you will obviously lose that appointment I would say, being a locum GP, that most practices are hard to get appointments at. I really feel for you, it's almost like you have to plan to be unwell. If it's something really important, have you tried speaking with the practice manager at your current practice and expressing your discontent?
Yeah if you re register before your appointment then you will lose the appointment x
I moved to GP at hand. It's online and 75% of people get an appointment on the day! Jump on, see how soon you can get an appointment if it allows you to see and then move. It's been the best thing I've done. I'm being seen soooo much faster than my normal doctors for plenty of different things! If you can't see, I suggest waiting til you appointment and then move