I would try repairing them before binning them and purchasing the Shimano upgrade. If there is no obvious fluid leak around the piston the seals are likely in good order and you've got a sticky piston rather than a failed calliper. This is a simple fix:
- Remove wheel and brake pads.
- Using a flat headed item hold the working piston in.
- Squeeze the lever, the non-retracting piston should be fully out.
- While holding the lever wipe around the piston with a clean lint free cloth.
- Still holding lever lubricate the retracting bit of the piston with a bit of brake fluid (DOT 4 or 5.1, not oil, as this may damage the seals and/or might work through the seals and mix with the brake fluid)
- Release the lever and repeat lubricating 5 more times, the piston should now be retracting.
- Wipe off any residual brake fluid from the caliper.
- Use a brake block to push both pistons evenly back into the caliper. Pull the lever with the block in to align.
- Refit the pads and wheel.
- Realign the caliper.
Please note, never pull the lever when there's nothing in between the two pistons. It's hard to separate pads and near impossible to separate pistons without bleeding.
Juicy 7s were good brakes, if they aren't broken there's no point rushing out to replace them.