Redmine

Before I describe my question, here's the required info I've been able to find...

Environment:
Redmine version 2.3.1.stable.677
Ruby version 1.9.3 (x86_64-linux)
Rails version 3.2.13
Environment production
Database adapter Mysql2
Redmine plugins:
no plugin installed

So, we recently upgraded our Redmine installation up to the above version from...well, let's say a very, very old version. Now that they are available, I've created multiselect lists in order to combine multiple existing fields, so that I can clean up our tickets and remove the many custom fields we had been using previously to capture data. However, I'm running into a major hurdle with moving that information over...

Is there any way to mass/bulk edit issues, either through the right-click convenience menu or through the bulk edit screen, in such a way that it retains the present value(s) of a multiselect field and adds the new value(s) I'm selecting? Everything I've attempted so far results in the existing multiselect value(s) being removed and replaced by the one(s) I'm choosing in the bulk edit, but I need to be able to keep what's there and add new entries, so that I can sift through existing fields and move selections into the new fields.

I hope that made sense...thanks in advance for any help.