How difficult would it be to replace these fuses with circuit breakers?
Yes you can get pop in breaker replacements.

They look something like this:  

The important thing is to ensure that you use a breaker that is the same ampage as your fuse (or very close to).  When I did mine (before rewiring the house) I had some fuse wire that was something like 15A.  But the closest breaker was 16A.  That's usually ok because the wiring is actually rated higher than that.  But for safety make it as close as you can.

Breakers on ebay in UK...