Is it possible to adjust KfW funding from 55 to 40?

We have a similar situation (KfW 55 applied for / old repayment subsidy amounting to 5,000,-). Now, before construction starts and we want to get a photovoltaic system including a storage battery, we fulfill other conditions...

Can I switch to another funding level after loan approval?
Yes:

[*]for loan amounts not yet drawn if
[*]the regulations for the start of the project are still met (see question: When can I start the construction work?).

Please explain to your financing partner your waiver of the KfW funding not yet drawn and submit a complete new application (including a new "confirmation of application (BZA)") for the (other) KfW efficiency house level.
The new approval will be issued according to the then-current product terms and conditions (including interest rate and repayment subsidy amount).
For drawn loan amounts or if the regulations for the start of the project are no longer met:

[*]a change to a higher funding level (e.g., from KfW Efficiency House 55 to KfW Efficiency House 40) is no longer possible.
[*]for a change to a lower funding level (e.g., from KfW Efficiency House 40 to KfW Efficiency House 55) a "confirmation of application (BZA)" at least technically corresponding to the original leaflet must be submitted. The purpose of use, the amount of the repayment subsidy, and, if necessary, the interest rate as well as the maximum funding amount will be adjusted based on the original product conditions.
A switch to KfW Efficiency House 70 is only possible for approvals with application receipt until 31.03.2016.

The possibility to change is generally granted by KfW only once for a project. This goodwill requires the consent of the financing partner.

Does this mean that there is actually a chance to switch?

Yes, if your loan has not yet been drawn down, you waive the old loan and submit a new application (with a higher funding level). And your bank also has to cooperate with this. They have effort and no benefit from it.