Best quality and quickest way to upconvert is hardware.
You have not stated your framerates. But SD 29.97i and HD 59.94i is the same thing (but the field order is different.
I you don’t have the hardware I would suggest paying for the upconvert, if you have a finished SD movie that should not be too expensive.
If you want to do it with software you want to upconvert the SD with compressor with Frame Control or with better plugins that have better methods of upconverting. I would suggest trying InstantHD. Red Giant normally has demo versions of their software.