I can't find the tabify/untabify commands. I'm sure it is my fault because it isn't possible that those commands can be missing from a programming editor. But if they really are missing, please add commands to change all tab s to spaces and vice versa depending on the IDE tab settings.

it is "Optimal Fill"
and is applied automatically when you leave the line.

No, it's not. It would be the inverse of that option. I always format my files with spaces, not tabs. But sometimes I cut-and-paste code from files that are using tabs. In this case I want the tabs to be converted to spaces to keep with my formatting conventions.

right. It re-indents current line with spaces or tabs depending on "Use Tab Character" checkbox state.

Umm, no it doesn't. With Use tab character unchecked and Optimal fill checked, try typing a few tabs. They will indeed be inserted as tab characters. No change when I leave the line or when I save the file.

Edit: Perhaps you meant that only spaces are used when Optimal fill is off? True, but that still leaves the problem with text that is cut-and-pasted from a file that does use tabs.

Probably we were talking about different things.
I thought you need to convert spaces to tabs.
You'll see it's possible if try the following:
-check View->Show hidden characters
-open Tools->Settings->Editor Settings
-check Use Tab characters
-check Optimal Fill

Now, enter a line of text, move cursor to the leftmost position, insert say 8 spaces, move cursort to a different line. You'll see that all the spaces are converted to tabs.

But seems you need to convert tabs to spaces
Yet, even you paste text with tabs when use tab characters is unchecked, it will not conver tabs to spaces.
I think it's fixable and we'll do so in build 4622.

That sounds great! I still would like to have Tabify / Untabify as separate commands though, since that makes it easy to change files from one convention to another when sharing them with other people. That is probably the easiest way to reindent files to use a different spaces-to-tabs ratio too, i.e.

1. Set Spaces in tab to original indent
   
2. Tabify file
   
3. Set Spaces in tab to new setting
   
4. Untabify file

But if you can take care of the pasted-text problem by making Optimal fill convert tabs to spaces if Use tab character is not checked that will solve most of these problems.

a little note Optimal fill does not change Tabs to spaces, it only may change spaces to Tabs. What is supposed to clear tabs is Paste command when Use Tabs is off.