Is it possible to retain clients who relocate or, for that matter, acquire new clients from places well beyond the comfortable driving distance from your office?
Yes it is. I don't see that the distance matters as such in delivering the service when it comes to your part in it.
However, I would recommend developing added value to your remote service so that it has an element of uniqueness in the eyes of the client, such as a monthly consulting service explaining the progress of his fiscal year with some tips on emerging opportunities and potential dangers... that sort of thing.
A service like that is unique in regards to what the client is likely to find in his new area and it would help you RETAIN this client.
Because on the client's side, there might come a time when HE feels the distance is a barrier as he cannot simply "drop by" your office for this or that. Clients' habits and routines are basically why accounting IS local - it's their desire of proximity in special situations that's on top of the list there.
So I would recommend adding value to your service in such way that this can then be delivered routinely via the telephone.
Best wishes Harry Kafka