I suppose the first thing we need to discuss is if you upgrade to 10 you may not be able to get back to 8.1. If you take precautions, you should be OK.
Second, you can upgrade now and have the Preview Build activated. There a many unknowns about how the process will work and folks at Microsoft seem to put out different procedures almost every day. The preview builds so far still have problems and the update process is not yet set in stone. You may want to wait a month and install the final version and you will have a year to get the upgrade without additional cost.