Upgrading stirgeman pants

So I found a few really cheap lv20 stirgeman pants in the FM, and I plan on trying to upgrade at least one of them to the lv70 version. What are the chances of me blowing a pair of pants up?

Higher than the chance of them turning into a level 70 or 90 pair.

Chances are it will blow up, or it will stay at level 30 for a long time. I got extremely lucky with mine.

My original pair took blew up. My second pair went 20 -> 40 then 40 -> 70

Super lucky was I.

I have 2 level 20 pairs in my bank, awaiting a comeback.