Lisa, PXLated
OK: I re-did everything as you suggested, and found an even better way to do this. (BTW, I am using 1.3.2) But first, some notes:
1) I restored the stock profile template and verified that it all worked as normal. It did.
2) I renamed my modified file to “ht_theme.php” and uploaded that to the same directory (default): When, I went to Admin -> Members and Groups -> Member Profile Templates to look for the newly uploaded file: it wasn’t listed. Also, I went to Membership Preferences to change “Default Member Profile” theme—the only option in the drop-down was “default.” Hmmmm.
3) I created a new folder called “ht_theme” with my ht_theme.php inside. I returned to the ADMIN and Preferences pages. My new profile showed up as an option: I selected it and refreshed the membership pages in my site. I got this error, plus a few others:
Warning: _load_element(): Unable to access ./themes/profile_themes/ht_theme/profile_theme.php in /imedia/users/hearthis/htdocs/HTsystem/modules/member/mod.member.php on line 6327
4) I renamed my new profile, inside the ht_themes folder , to “profile_theme.php” per the clue in error above—this time everything worked.
So, the process is this:
1) Copy the profile_theme.php file and place it in a new folder alongside the “default” folder—the name of the folder will be on the list on the Preferences page, and, will show up as a **complete** new set of template fragments under an option of that same name on the Membership Profiles menu page. All of the fragments are automatically generated each time a new theme folder is added in the membership section.
2) Do not edit the “profile_theme.php” script as I did: this is where I made the second “discovery.”
Instead, in the Membership Profiles fragments listings, there are fragments like HTML_Header, HTML_Footer, Page_Header, and so on: there is (!) one of these for each of the functions in the PHP script. Of course. Edit those fragments, and the result is exacly the same as editing the script, except it’s “correct” way and much less hazardous. Does this make sense? I hiked ten miles to cross the street today.
))
I can now switch between the two memberships themes by choosing the applicable theme using the Membership Preferences/General Configuration drop-down.
In summary, in 1.3.2, you can simply edit the default theme fragments (HTML_Header and HTML_Footer, and maybe Page_Header, etc. if you really want to fine tune) and quickly re-skin the entire membership section. As you’ve suggested, by first making a new theme folder and placing a copy of “profile_theme.php” in that folder, you can choose between the stock and modified themes for application in membership section, with the benefit of having a backup and a clean upgrade path.
That’s it—and again, thanks for the help today. I learned a lot. 
Dave