You would have to get the agreement of your current IP to stand down, and find a new IP willing to take your case one half way through. Unlikely to be tempting to another IP as a "one-off", and even then a meeting of creditors would have to be called to effect the transfer, or an application made to Court. I have seen it done, but this is quite rare, and can be costly as Foggy has already mentioned.
Which firm are you with and why are you unhappy with their services?