Can I change the plugs on low-voltage lights to allow connection to dimmable outlets?
Assuming by this that you mean UK standard 13A 3-square pin plugs and 5A 3-round pin plugs then yes.

I have done this for my lights for several years.

You do have to bear one thing in mind.  While 13A square-pin plugs contain a fuse within the plug, 5A round-pin plugs do not.  They are protected by the circuit breaker at the main fuse box.

I assume your 13A plugs actually contain a 3A or 5A fuse if you're using them just for fairy lights.  Check the ratings and where your round-pin sockets are actually protected.