I'm kind of confused why the option is given to switch between versions of brush sets rather than new ones being added each version and ones that share the same name get replaced.
Is there a difference between say a 2b pencil from painter 12 to 2021? Also I'm curious if this is just a 2021 version thing or if each new version gives the option to use older sets
Generally all old brushes will work OK in any newer version of Painter; but may not operate exactly the same; as newer version may have different capabilities; especially with P22 and newer layer and paint engine compatibilities. Usually brushes paint the same but can be altered to use newer features.
The reverse most likely not true....many new brushes won't work in older Painter version because certain brush characteristics are not available in the older version; and sometimes they cause system problems. NOT recommended to use newer brushes in older versions....unless brush provider states which previous version is OK.
Then again Corel has often renamed and/or rearranged the brushes in different brush categories and/or libraries from version to version.
Best is to use the most current brush with the most current version of Painter.....for any/all Corel supplied default brushes.
As you may know, up thru P21, Corel supplied all old brushes. In P22 they did not; but did allow for free downloading and installing older default brush libraries.
There are many new (purchased or free from others; not default supplied with Painter) that are specific to a Painter version; you can use in a newer version but best not used in older version.
The only best way to know is to try an old brush and try a new (same name or very similar name) brush side-by-side and compare...use the one you like best or keep both if different.
Frequently old and new brushes with same names paint the same...so keeping old brushes may be duplicative and just use extra file space...so save any old ones (that don't seem to have a newer version, and you use them) in your own brush library/category and don't load libraries of old brushes (slows down the program startup time) unless you need them....and load them then.