No, you don't need to completely change the compost each year. Though it is a good idea to add some appropriate fertiliser or mulch to ensure that there is plenty of nutriments in the soil.

I usually take out the top 2 inches and put fresh compost in and then fork it in. I also sprinkle some dry fertilizer on top from time to time and the rain washes it in. I don't know if this the correct thing to do but it seems to work for me.

I find cutworms sometimes like to burrow in pots and if I don't change the compost they eat the roots and the plants become very sick. (I think they are cutworms - they look just like cream caterpillars with brown heads and haven't found pictures of them in any gardening books so far) So I think it is well worth tipping the plants out and having a good look, even if you only replace some of the compost.

Cream-colored, C-shaped worms with brown heads? Probably grubs. I always change out the compost in annual pots to prevent diseases from building up. Not practical with anything perennial, of course.

No Tugbrethil, they are not the little C shaped ones but much bigger, exactly like caterpillars and they wreak havoc. The results are a bit different from vine weevil attack as the first thing you notice with that is that all the roots have gone and the plant has had it. These caterpillars eat more slowly so the plant looks sick for quite a long time before it dies so you have a chance to get rid of them.

Whew! Glad we don't have that kind here! The cutworms here are green or brown, rest coiled in a spiral underground during the day, and eat plants' stems and leaves (not roots) at night. The several species of grubs we have here work slowly, as you described, not like vine weevils.