That doesn't sound substantiated at all. We are talking about a small frequency shift of e.g. a few 100 Hz.
But anyway, to understand the impedance matching requirements, you need to measure the complex transducer impedance over frequency for different load cases.
If you want to keep the frequency for any real or imagined reason, you'll use variable (e.g. stepwise switched) matching capacitors and inductors, or just accept a certain mismatching. Simply your decision.