Having never been involved off shore oil rig construction I am not presenting but the most general construction view.

All modern designs have a certain life expectancy, an overbuild safety factor and weak points with minimum overbuild. Old oil rigs may or may not have a reuse potential for any thing more than subsurface sheared anchor points for marine habitat.

It could also be that drilling rigs will be specifically designed and new built for geothermal offshore drilling. If the costs can be reasonable.