Is it possible to upgrade my older Velit Air Conditioner to have bluetooth capabilities?

In general, no this is not recommended, although it may be possible. This is because although the display planel with bluetooth capability looks the same, there are likely different components inside that require different firmware on the display board. 

So in the process of switching to a bluetooth control panel, you could have other issues with the Air Conditioner unit otherwise not performing as it did before the panel swap due to mismatched components/firmware - resulting in the AC unit being disfunctional when it was otherwise functional before.

We understand this can be disappointing, but it's also difficult for Velit to inovate as a company and improve our products, and ensure backwards compatibility always. It's a tough balance for us to innovate and add features or improve the products, while not upsetting customers who purchased earlier before those product nehancements are released. So we must make decisions about improving our products vs. ensuring backward compatibility with retrofitted upgrades like bluetooth capability.

Typically, we are not looking to source cheaper parts, but rather more reliable parts that perform better. This means that older units could have components like compressors that do not run on the same firmware as the newer bluetooth enabled units for example.