My experience being, if the new frequently flyer is used, then the Gold status of the old frequent flyer Gold status will not be reflected.

I was able to use the lounge by showing my old frequent flyer gold status card, albeit some discussions will arise in most lounges (e.g. IST where plane ticket is used for lounge access).

However, I have not been successful in getting the additional luggage benefit by flashing my old frequent flyer gold status card.

Qualifier: I tried switching when Thai changed its flight redemption requirement (i.e. pre-Covid times). However, I did not follow through as I needed the additional luggage space.