Was there eventually a vaccine for sars or mers, and could that vaccine be adapted to covid-19 if they just swapped out the strains in the vaccine?

COVID vaccine: There are no generally available vaccines for SARS or MERS, but certainly there was research that could have potentially led to vaccines had those outbreaks lasted longer or affected more people. That work probably will accelerate research on a COVID. But to my knowledge, there are no MERS/SARS vaccines that could be used.

Vaccines: I don't think that there were approved vaccines for SARS although there was a clinical trial for an inactivated form of SARS-CoV & pre-clinical studies for several live attenuated vaccines. REF: https://www.ncbi.nlm.nih.gov/pubmed/16035944. Per the German Center for Infection Research, there is currently a human trial for a vaccine against MERS. REF: https://www.sciencedaily.com/releases/2020/04