Hi, Jenny! Guests of Walt Disney World Resort hotels are able to customize MagicBands
to be sent to their home at no cost prior to their visit. There is a good selection of basic colors from which to choose (I am still holding out hope for a purple one!). At this time, you are not able to upgrade those MagicBands. However, you can purchase additional MagicBands during your vacation at many merchandise locations, at which time Guest Relations can assist you in linking the new bands to your My Disney Experience account. The pricing of MagicBands does vary, but most I have seen range from $12.95 - $24.95.
Since you are interested in more galactic styles, you can visit several places to find the MagicBands you are looking for, including
Darth's Mall,
Tatooine Traders,
Mickey’s of Hollywood,
Beverly Sunset and Studio Prop Shop. I love the Stormtrooper themed MagicBand seen in
this recent Disney Parks Blog post. There were several limited edition styles released at the beginning of Star Wars Weekends, but I believe all of those have sold out. The open edition ones, such as those in the Disney Parks Blog post linked above, do not have limited quantities. I did see those at Disney's Hollywood Studios merchandise locations this weekend.
May the Force be with you during your vacation!
Brandy