I have been upgrading a Power App that was provided to my department. It has been a lot of "invested" time to upgrade button by button, if there is an option to use new controls, there should be a direct way to upgrade them without adding one by one and copying and pasting the same code for the onSelects.
This is a good example:

We have the classic and the "modern," but considering that this is HTML, the CSS should be able to be applied without you creating button by button and replacing them one by one.