We’re trying to reduce our energy costs. Would changing the storage temprature from -80°C to -70°C have a concequence on the quality of our samples?

This could be, and you could do a validation study for this. We refer to the following sources:
Review on long-term storage temperatures: https://www.liebertpub.com/doi/10.1089/bio.2013.0084
Lukasz Kozera BBMRI-ERIC webinar series on pre-analytical quality: https://www.bbmri-eric.eu/services/e-learning/
BBMRI.de white paper: https://www.bbmri.de/fileadmin/user_upload/PDFs/GBN_Recommendations_Energy_Saving_-80-ULT_Nov2022.pdf