Possible subsidies when replacing an old oil heating system?

do I have to submit my applications before the first gutting measures?

Always submit applications before the start of measures (except energy consultant).

And HOW is it subsidized now? Could you get a loan from KfW and only have to pay back 50%? Is that true? Yes or no?

It really doesn’t help, you either have to find the best subsidy for you yourself or with an energy consultant.
I only carried out the individual measure of heating system renewal. That is only available from Bafa (not from KfW). And with Bafa it works like this: you apply for the subsidy amount with the offer, then implement it, then initially pay EVERYTHING yourself. Only after completion can you submit all invoices and get X percent of the total amount transferred back to your account. So yes, you have to have some extra cash.
For this specific individual measure, there is indeed, in my case and probably also in yours, 50% on everything related to the heating system. Complete disposal, piping, electrical work, sealing holes, control, ventilation, and also underfloor heating including screed. (The complete list is easy to find.)
And if you decide to go for it, you CANNOT finance the whole thing through KfW. At least not with subsidized loans.

However, if you fundamentally renovate your entire house and want and can achieve an energy efficiency level, financing through one of the KfW programs might be worthwhile. Repayment usually happens through repayment grants on the loan.

The websites of KfW and Bafa are really understandable. You just have to read a little.

I only carried out the individual measure of heating replacement. This is only available from Bafa (not from KfW).

@ TE: I am a different example and am renovating my two-family house into a KfW 55 house. I get 40% reimbursed for heating, screed, insulation, disposal, flooring, among other things. I finance EVERYTHING from equity upfront and then receive a cash grant from KfW after the completion of the measures.

@ TE: I am another example and am renovating my two-family house into a KfW 55 house. I get 40% reimbursed for heating, screed, insulation, disposal, floor coverings, among others. I finance EVERYTHING from equity upfront and then receive a cash grant from KfW after the completion of the measures.

What about new windows? Do you also get 40% for those? And the electrical work?

What about new windows? Do you also get 40% there? And the electrical system?

Yes, windows also 40%. Electrical work is not an energy subsidy - there is nothing for that. At most for the ERRs, but I can do without them.