Unfortunately, due to the current nature of the issue, there are no experiences to read about.
As already written here before, it is the same procedure as with the last VAT change.
Basically, it is like this for you:
If the construction company actually changes the contract retroactively (i.e. after 06/30/2020), agrees on partial services and settles with you accordingly – i.e. with 16% VAT – and no reservation of a subsequent claim of an increase in VAT to 19% by the tax office is agreed in the contract (if this is even legally permissible under civil law), then you are off the hook. The entrepreneur owes the legally correct VAT.
For the entrepreneur, the following applies:
If the partial services were agreed upon before 07/01/2020 or the agreement was changed to partial services before 07/01/2020, the tax office generally accepts this. Therefore, 16% VAT applies for all partial services rendered and accepted between 07/01 and 12/31/2020.
If the agreement was changed after 07/01/2020 until 12/31/2020, it depends on the reason for the change. Here, the tax office initially assumes that this is done to save taxes. Therefore, this would be rejected according to § 42 AO unless you can provide a plausible, understandable reason for the subsequent change of the agreement. I cannot think of a reason here, other than tax savings, for the client (builder) to agree to such a change. Keywords here are transfer of risk and start of warranty.
The forum naturally wants and must never replace legal advice.
The question is only what you, as a private person, hope to gain from such advice now? In my opinion, the entrepreneur needs the advice here, and a good one, if he does not want to end up as a loser in the next company or VAT special audit.