It might be worth trying to free the motorised valve before you replace it, by giving the moving parts a squirt of lubricant (WD40) and sliding the lever back and forth a few times. If this works then you may be able to postpone replacing it until after the heating season is over.
If the valve is readily accessible then replacing it is not a particularly big job. You may be able to achieve this without draining down the system, by fitting rubber bungs in the cold feed and expansion pipes - at the header tank. Bungs for this specific purpose are usually available from plumber's merchants (and possibly B&Q). Provided that both open vent pipes have an airtight seal, you may only need to catch/mop-up a couple of cupfuls of water when you remove the motorised valve. Otherwise, you will need to drain the system down to a level below the motorised valve.
How difficult it is to remove the old valve depends on how much 'give' there is in the pipework. The pipes go into the valve's body about a centimetre or so. Hence, you with have to move the pipe by at least that amount to get it out. Some people reuse the old nuts and olives when fitting a new valve, but it's best to replace them (you can get the old olives off by part cutting through them with a hacksaw and then splitting them with the twist of a screwdriver - inserted into the cut). After fitting the new valve, remove the bungs, check for leaks and bleed the air from the vent on top of the pipe that connects to the upper end of the cylinder coil. If the pump is on the same level as (or above) the motorised valve, then you will have to bleed it as well.
All that remains then is to wire in the new motorised valve in place of the old one.