1. No. Your new downloaded SAS depot will be a new version of SAS software must be stored in its own separate folder.
Regarding 2 and 3: are you doing an upgrade in place or are you going to do the install on new servers?
If you are doing an upgrade in-place and haven't changed your server designs then nothing will need to change except ensuring that any customisations you have done to configs and autoexecs are retained.