Hey there. There is unfortunately nothing else for you to do other than buy the new modification. Quiet some time ago the MU system has been changed. Now you cannot MU the lower modifications once you reach a level at which the new modification unlocks. I would suggest you to not MU low modifications anyway. The difference they provide on lower Mks is pretty insignificant, especially if you play regularly and/or during the boosted experience days that are fairly common these days.

Hello. For some time now, it’s impossible to microupgrade lower modifications once you reached the rank on which the higher modification of that particular equipment is unlocked.

The link sends you to the Help Site topic of Reporting a Player. I see you commented on that post some months ago. My guess is that you clicked on "follow" button, and now you are getting email notifications whenever someone posts new comment in there.
If you  want to avoid getting any more emails, log in again and unfollow the post.

Hey there. If you would like to potentially win some promo codes yourself, you can try your luck by participating in the Promo Code giveaways hosted by the Tanki Online eSports team that take place during the livestreams of eSports matches. Just follow the official Tanki Online eSports channel. Follow the channel or subscribe to get notified when the next match takes place and then simply enter your nickname into the Google Form during the match. The commentators will always inform you in the livestream when the Google Form is opened for submissions. The link to the Google Form is located always in the description box of the video.