Unless you are OK with undercarbonated beer, I would re-prime. It is going to be a bit of a PITA, but here is a method that works:
1. You absolutely MUST let all the current priming sugar ferment-out before you re-prime. If you don't, your resulting carbonation will be unpredictable, and could result in inconsistent carbonation from bottle to bottle, and possibly even bottle bombs. Move your bottles to someplace warm (say 75F) if they aren't already. They need at least a week (two is better) at warm temps to carbonate.
2. Once you are certain the original priming sugar has fermented away in the bottles, carefully vent the CO2 out of each bottle, but don't remove the caps. If not much comes out, try a little agitation to get that CO2 out of solution. You want to get as much out as possible.
3. Cool the beer as cold as you can get it (without freezing, of course).
4. One bottle at a time, remove the cap and add a Cooper's carbonation drop or the appropriate amount of Munton's carbonation tabs (I have used both and *greatly* prefer the latter) to each bottle. If you didn't vent all the CO2 out and/or if they are not cold, adding the sugar tabs will cause an instant gusher. Don't say you weren't warned!!!
5. Using a new sanitized cap, reseal the bottle. Give it a gentle shake to ensure that the bottle is sealed and to resuspend the yeast that may have settled to the bottom of the bottle. Repeat steps 4 - 5 for all remaining bottles.
6. Move your bottles to a warm spot (70 - 75F) for 2 weeks minimum.
Hope that helps!