I've not disassembled either lenses (the SLR or the RF version), but are you 100% sure the front elements are swappable? Kiitos, the Nikon experts in Japan, said that it was not something simple to do and possibly not something you could do at all. The front elements are obviously the same, but it seems they are mounted completely differently.
How does having replacement front glass affect anything? As for already having a 2.1cm, that's true, but I won't be keeping two of them. I am for sure not a collector!😀