you only clean the system between seasons.
As for draining the nutes without moving the plants, I use one of two methods now.I like to put a drain near the bottom of the tote. It might leave 1/4" of liquid in the bottom but that doesn't bother me. if you system is set up this way, you could put a valved redirect in your pump line, then use the pump to remove the old nutes. That generally leaves a little more nutrient on the bottom. But again, does it matter?
Other options are let the plants deal with it. move them as needed and drain the reservoir or use a siphon. both these methods do an excellent job of emptying out the reservoir, but can be a pain in the a** and sometimes don't taste good.

It kinda depends on the method you are using.

I use DWC a lot and just resupply what the plants use and NEVER change out the nutrient solution except as grizzman says -- when I switch out the plants.

I grow indeterminate tomatoes mostly and some of them have been growing in the same little bucket for 3 years! I glue aluminum foil on the outside of the buckets to prevent light from forming algae. Some forms anyway, but it doesn't seem to hurt anything.

I'm currently working on a ''shelter" which will house a row of 2-gallon plastic buckets in a wooden cave to prevent light from forming algae.

The idea here is to have more room for the maters to spread out and grow more. I've up to now used the "single-truss" method of pruning the plants, but of late have decided to just let them grow like a massive monster. That will take more room than I have in my little greenhouse, so I've decided to move things outdoors and hence, the wooden cave idea. Each bucket has its own air pump, by the way.

Took about 6 hours over two days to change out all the nutrient. Think I'm going to change every 4 weeks or if PH changes after 2 weeks.