Velocity rims/wheels are generally very nice. No reason to replace them if there aren't any problems.

650c is a larger wheel diameter than 26". Sometimes bikes can accept both rim sizes, but often not so much. It generally depends on the brake set up.

Many mountain bike rims, the lighter xc variety, do just fine with road set ups. It is about rim width and the ability to mate with brake shoes. There are a lot of good 26" rim options that will work with road brakes, including the Velocities.

One thing to note: 650c tire options are limited to 23c or narrow (i.e., a very narrow racing tire, usually). Terry sells one okay quality tire that is 27 or 28c, that is designed for more versatile usage, but that is IT as far as 650c offerings wider than 23c.

If you want a versatile tire selection, stick with 26". You can get a wheelset as light as 650c, and it will allow a lot more versatility in tires.