Good idea, and it works to some extent - however, the styles are not completely consistent because GigaLem added pieces to his modifications that your versions don't have.
This by itself isn't a problem either - I just opened gigalem_metalmd in the old graphic set tool and added the missing indexes to the translation table, referring to their respective counterparts from the converted version. Those were quite a lot of additional pieces for metalmd and purplemd, though, so lots of unnecessary work, because I'm sure translation tables for these styles must already exist somewhere!
More importantly, though, gigalem_metalmd in Old Formats had two terrain pieces both named "alt9" - one a horizontally-oriented block, one being a long vertical piece.
The former is missing from New Formats, the latter is what is named "alt9" in the New Formats version.
This is not just an optional exit, this is an actual missing piece of terrain, which, as a consequence, can make converted levels in this style not work in New Formats.
Please, GigaLem, re-add this piece of terrain! (I've exported it with the old graphic set tool and attached it so you know which one I mean.)
I think when you did some re-shading of those "building"-like pieces, you simply forgot about this one.
All of this is a lot of unnecessary nuisance - it would be way easier if you had simply shared the missing translation tables for gigalem_metalmd and gigalem_purplemd.
After all, in the instructions on graphic set conversion, updating the translation table is officially mentioned as part of "the job" of whoever converts a graphic set! I provided mine for my "Instruments" and "Autumn" tilesets as well. If one is not willing to provide the translation tables (GigaLem did do so via PM to me for desertmd, but not for metal and purple), it would actually be more helpful not to do a conversion at all. Because then somebody else can drag the original .dat-tileset onto the converter, and in the case of these tilesets, this would have been a lot easier than adapting namida's translation tables to the new styles, because all the pieces already had proper names in the Old Formats version, so everything would have fallen into place correctly.
However, it was not possible for me to do that because that would have created an
alternative version of these styles - meaning: A
complete one, whereas the New Formats versions of desertmd and metalmd lack some pieces.
The fact that objects as well as terrain were removed from the New Formats versions of these tilesets makes converting levels which used these tilesets similarly error-prone and annoying as those created with the last three Gronkling tilesets, which were split up into two tilesets each in New Formats.
For Gronkling, I just recently did the remaining translation tables; I hoped this would be easier for more commonly used tilesets such as GigaLem's Lemmings Plus edits (weren't they even so popular that they were considered to become the new standard versions of those tilesets at one point?).
Hence, I think the fact that there was no proper method of converting levels in these tilesets until I noticed this is a bit of a shame.
I've attached my edits of the translation tables, I've done the work to the best of my ability, but some slight readjustments may be necessary. For example, the backgrounds don't get selected properly yet, and the exit_dirt from the purplemd tileset needs some repositioning, although it doesn't occur to me why this occurs.