I'm planning to change the rims from 27.5 (ERD=566mm) to 27.5+ (ERD=563mm). Will I be able to reuse the spokes in this case, or is it worth getting new ones?
It is completely normal to replace a worn out rim by transferring the existing spokes to a new rim - it's slightly easier than relacing the wheel, since you can tape the rims together and transfer each spoke one by one (slacken off the tension on all the spokes of the old wheel before you start!). A 3mm difference in ERD will correspond to about 1mm in ideal spoke length - unless the existing spokes were too long (ideal is top of spoke at bottom of screwdriver slot) this shouldn't be a problem. Washers under the nipple heads (eg Sapim polyax) would help compensate if the spokes are on the long side. Going the other way (smaller to larger ERD) too-short spokes can be compensated for with longer nipples - although for a 1mm difference I wouldn't bother.