You always do a backup. I have heard of power failure & OS crashing during updates. If You come here You get news of unbelieveble bad and good coincidences.
Yes, all 1.5.x versions retain history. Or rather a new install does not overwrite the profile files in a used Profile.
With default settings both Your 1.5.x will use the same profile(s) and data in %apdata% - unless You log into PC with different user names. The defaults are in %user name%%apdata%.
I am not sure what You want to do with Your two K-Meleons. But:
The easiest way to use the same profile as Admin & xps - de-select multi user support. That puts profile into install folder. You grant writing rights to any of the identies to ./K-Meleon/profiles/.... voila.
I use that setup for myself on a multi boot machine - across 2 different Oses and several personae. That setup is also used for testing. Independent profiles in each K-Meleon install set up that way.
You can change things later.
By default all Profiles internally use relative paths to their K-Meleon's files.
Result: Especially with 1.5.x versions even complete Profiles can be copied to new locations.
You can even join a profile in %pdata% to a portable, if You want that.
All it takes is that You either edit profile.ini - Else (even easier) You let create K-Meleon a profile and copy the old, existing data into it.
p.s. this is not Firefox or Tbird that destroy an old profile after an exident that only damaged profile path information.
Worst case scenary is normally that K-Meleon "forgets" his profile location and cannot find data in old location. PANICed PPL come here: I lost my bookmarks...
You fix that with the yeast of this post.
Bookmarks have automatic default backup - for other profile data backup several methods even extensions exist.