@Logic I experienced the same problem when using MC to convert BMS 4.34 missions to BMS 4.35.
(See the ugly history of that “dialogue” , if you wish, but it may not be enlightening…)
What this boils down to, per Falcas, is the presence of the proper “Conv*.txt” files in your MC/ Conv/Korea’ directory. Falcas quickly provided an update, and after that the “automatically replace” option worked smoothly.
However, as far as I can see, the very latest release of MC is 0.5.19.667, (which, although it dates to 25 Jan 2022), when installed and run once in the “Convert Mission” mode, seems to leave an empty “ConvTF.txt” file in the above Conv directory. (Well, empty except for its header line.)
When I attempt to use MC to convert a 4.35 mission, it produces an endless series of Caution dialogs (of “Could not find objective…” type).
The “TranslateObj.txt” file is dated 1/28/2021.
I could be completely off the mark (and I’ve been upbraided for that before), but I’m not convinced that Falcas has completed the updates needed to the conversion files…
Best I can see, a PM to [Falcas] is likely to get a very timely fix! He has always been incredibly responisve to problem reports on all of his splendid BMS tools!