2.7.3a? Yikes, that's pretty old
Before migrating to 2.5, be aware that you must update 1.5 to the latest version of DT Register FIRST, then migrate the data. Instructions will be in the current package.
Also, make sure you don't have our JomSocial plugins installed/enabled if you don't actually have JomSocial installed. Beyond that, you just need to update as you WILL have issues running that very old package.