Designator I don't think usually changes (for...obvious reasons?).
You might be able to reset it via annotation. The annotation dialog lets you control which components get assigned, reset, sorted, etc., and over what numerical ranges.
You could also select them manually (via SCH List or Filter query), and use the Inspector to find-and-replace the string. (Select the components or their designators, I don't think it matters -- they share the same common value. Though they call it a different parameter (Name, or Text, or.. something like that, I'd have to check really).
A replacement string is written:
{was=becomes}
You can have the selection:
RV4, RV9, RV23, ...
and on the Designator = "..." row, enter the string "{V=}" (no quotes). "V" is replaced by "", and now you get
R4, R9, R23, ...
To add it back in, you'd write {R=RV}.
Only strings which contain the substring are affected, so this comes in handy for certain renumbering activities.
You can list multiple substrings; they are evaluated in order I think. That looks like,
{1=2,2=3,3=4}
replaces all instances of "1" with "4" (rather useless, but if they weren't chained, obviously, it would be effective). Or you can write out
{1=2} <ENTER>
{2=3} <ENTER>
etc. Same thing.
There's also some functions that can be used if you bring up the "..." dialog (specifically, they are a subset of Pascal string functions -- you'll have to look up the list to see what Altium actually supports). The Copy(field, offset, length) function is probably the most useful (e.g., if you want to make a Part's Comment include properties like X1/Y1 position or...!?), or of recasting part of the field itself (i.e., for the designator's Text field, use "!" to reference itself, then cut down to the numerical part, and add/subtract an offset, to shift a bunch at once).
Or you can brute-force it by selecting the parts in the List, turning on "Edit" mode, and pasting in an array from a spreadsheet.
Aaaaaaaand, I'm sure Free_Electron has a selection of scripts or tools for such things handy... which I still haven't checked out yet, to my dismay I'm sure!
Tim