Feature: tag - tag renaming

It would be nice to have tag-tag renaming as of filename. Currently it's not possible to deconstruct tag that it could be possible to create a new one. In example: TESTME (RANDOMGUY REMIX) -> Testme (Randomguy remix). Now I have to 1. make a filename for decostruction, then 2. reconstruct the filename and 3. convert filename to tag.

You can use
Convert Tag -> Tag with the field
_FILENAME
and construct it as wanted.

BTW from the help:

There are two special field names, _FILENAME and _DIRECTORY , which target the file name and the directory name, respectively.

Read the provided scenario at first please..

I you expect further help, it would help if you create an example that is reproducable, please...

If I had to guess: Could it be that you want adjust the case spelling in the field TITLE from
old:
TESTME (RANDOMGUY REMIX)
to new:
Testme (Randomguy remix)
?

If yes, there are two ways:
a) You can use an Action of type "Case Conversion"
or
b) You can use one of the $caps string functions

As I said read the provided scenario first

TBH I do not understand the "scenario".
If you you want to split a field (is that

?)
then please have a look at an action of the type "Guess value".
It seems to me that you are looking for a function that provides a preview like the converters do also for "Guess value".
The described detour from a tag field to the filename and then back into fields is not necessary with "Guess value" and the detour also has the disadvantage that a filename has a number of invalid characters that may appear in the tag data without any problems. SO these characters would be lost.

What's that? Where can I find it?

See the documentation:

It's for filename - tag conversion. Not tag - tag (in my case - TITLE to TITLE).

The Source Format in "Guess value" takes any format string, so it is not limited to the filename.

See e.g. this thread for an example

(on a Mac "Guess values" is called "Import tag fields")

I think it's too far from the problem. Let's make it easier. I have TITLE (%title%) and I want to make new title from it. Let's say from form $1 $2 ($3) into $2 - ($3) $1. Easiest way is to pass it to filename then deconstruct (or decomposing, choose) it to tags and concat these into required pattern. Do you know easier way?

Without a real example, just a theoretical guess:
Convert Tag -> Tag
Field: TITLE
Format string:
$regexp(%TITLE%,(.*) (.*) \((.*)\),$2 - ($3) $1)

A real example would be very helpful here. Several regulars have attempted to provide support but your website sponsors haven't filled in the gaps.

The Guess Values action is what I would suggest as well. But without actual data to work with I too am really just "guessing" at what is being asked.

That looks a lot like a task for a regular expression in TITLE in an action of the type "Format value" or Convert>Tag-Tag
Taking TEST ME (RANDOMGUY REMIX)
Format string: $regexp('TEST ME (RANDOMGUY REMIX)',(.*?) (.*?) (\(.*\)),$2 - $3 $1)

leads to: ME - (RANDOMGUY REMIX) TEST

True. It's just an example because any naming can be found in filename/tags.

A filename is no tag.
As you have seen a lot of other users in this forum all came to the same result: use a regular expression.
Together with Convert>Tag-Tag where you get a preview, I would say that the feature is already there.

It's easier to pass required tags into filename and back because different algorithms are used than construct regulars all the time.

As I said before:

But of courseyou are free to apply a function of your liking.

I do not understand what you mean by

as regular expressions describe patterns just like the converter filename-tag does, I do not see where the disadvantage of the regular expression should lie.
On the contrary: as you can manipulate the greadiness and limit the patterns much better to data types the regular expressions are much more flexible.
I admit that a converter may be easier to set up for the layman the more flexible and powerful function cna be found in a regular expression.

Too many devs here. I pass. KISS