You need to do SOMETHING to stop the boiler water coming out, like draining, bunging or freezing. No the HW cylinder though which isn't connected unless yo have a primatic.
Once the body is off the head you can inspect the rotor. If it's gunged up then oilman's comment is right.
The pump nuts (the 2&1/16th inch ones) can bind on the pump flanges and make it hard to get apart. Yo don't have to do so if you're just changing the body. If you DO have to change the whole pump, it's a lot easier getting hold of the pump head than the whole thing (big pair of grips).
If the pump nuts really bind,cut and split them off.
If your valves are that bad and you care about it, you would be replacing the valves anyway, so just undo the 22mm compression nuts on the valves and dump the lot.
Gate type valves are better than the 1/4 turn ones as a rule but they're a different length, so won't go straight in, usually.