why remove 1.15 revision?

I'm not sure what you mean by 1.15 revision, but if you mean the dev release for version 1 of markItUp, then I clearly haven't removed it - it's right there on the project page. There never was a non-dev release of markItUp! to the best of my knowledge.

If version 1 is working for you, by all means keep using it - especially since there isn't any sort of upgrade path coded in to the version 2 module yet. But for new installations, I recommend using the version 2 module; it uses less code and is more Drupal-like.

Well, you could uninstall it as normal (not just deactivate it, but uninstall it), though it doesn't properly delete all the variables it creates. But as I said, if it ain't broke, don't trip over yourself trying to fix it.

Then uninstall version 1 and install version 2. It won't break anything (I don't think), but you'll have to reassign your sets/skins and uninstalling version 1 doesn't completely delete all the variables it creates.