If re-encoding is the last thing you want to do, then re-importing is definitely the *second* to last thing to do... ;)

iTunes isn't "über-intelligent" but it isn't that dumb either.
If you replace the original file with a new one, iTunes will accept all the new tags from the file but it will leave any existing ones intact, with the exception of the cover art which always has to be written directly *into* the mp3/AAC file.

In other words:
Let's say you have a corrupted MP3 file which shows up in iTunes with all its tags and cover art but it doesn't play right.

You repair the MP3 with MP3 Trimmer which strips *all* metadata (ID3 tags incl. cover art) from the file, leaving only the blank audio file.

Then you replace the old file with the repaired file. By "replacing" you indeed have to actually *delete* or at least rename the old file, while giving the old name to the repaired file, otherwise iTunes might still track the old one; even in Trash.
iTunes will now still show all tags and existing track info for the repaired file except for the cover art. The iTunes metadata are not written to the ID3 tags of the file automatically, however. You need to change (temporarily) at least one tag in iTunes to do so, e.g. by adding a character (e.g. a space) to the track's name and then deleting it again (which you can do pretty fast by selecting the track in iTunes and pressing Enter > Down Arrow > Space > Enter (now the ID3 tag gets written to the file) > Enter (again) > Down Arrow > Backspace (to remove the extra space at the end of the track's name) > Enter). That will write all compatible tags back to the file except for the cover art which you need to add manually (or by using any of those hunderds of helper apps/AppleScripts out there).

Note that using the Convert ID3 Tags command from the iTunes Advanced menu will write all tags *except* the Track Name tag, if the MP3 file didn't have any ID3 tags at all. (If you ever use that command, make sure to select ID3 Tag v2.3!) Again, the Track Name tag will be written only after you have altered it using the method described above.

If you only *edit* an MP3 file using MP3 Trimmer without stripping the metadata, all you need to do is simply replace the original file. No need to reimport either, iTunes will treat the new file like it were the original file, only updating its new length.

Trust me, I've done it more than thousand times already... :)
(I'm DJ, so I need to alter MP3s quite often.)