spanners / wrenches - to turn both nozzles (common sizes are 6-8 metric wrenches)
spanner / (fitting or adjustable) wrench - to hold a heating block
or small vise - to hold a heating block (if you are going to unmount the hotend for operation), with non-flammable soft-jaws,
screwdriver(s)/Allan-keys - when you need to remove shroud or plan to ...
The exact procedure depends on the design of the printer.
After unloading filament, remove and install the nozzle at the highest temperature you will run. Otherwise, you will not get a good seal between the nozzle and heat break if you depending on tightening at lower than operating temperature. A bad seal allows the filament to leak out (oozing). ...
Based on CNC Kitchen measurements,
the wear can be initially estimated by judging the overall length of the nozzle. Abrasive particles wear the nozzle very little on the channel (so the diameter doesn't change, for a while) but they wear out the outer surface in contact with the printed part, resulting in a shortening.