I was recently looking for subtitle conversion tools when I ran across these:

http://konwerternapisow.pl/en/

and

http://www.subsedit.com/convert

konwerternapisow converts to .txt (MicroDVD) .sub (MicroDVD) .srt (SubRip) .mpl

I tried a .sub (MicroDVD) to .srt and it worked perfectly.

The subsedit site didn't seem to work (for me)...maybe it didn't like my browser.

Anyway, hope this will be helpful to some.

KD