Is it difficult to move firms as soon as you qualify?
Quite a lot of variables here to say how difficult it is.
There are enough people who do it upon qualification though, either because they want to move away from the firm they trained in or because they aren’t offered a NQ role.
But how likely it is will depend on the firm you train with, the work you have done as a trainee, the seats/secondments you have done, and where you are aiming to work for upon qualification (both in terms of firm and practice area).
It isn’t straight forward and will depend on other factors such as the networks you build/the agencies you register with/how proactive you are in seeking a NQ role too (which can be difficult if you are busy working).