Hey Archie!
Nice head shot! You are doing it the RIGHT way-this is the only logical way to remove your halyards for inspection/replacement, and then re-install them (or the new ones).
Per the other idea, sure: If you think they might be a bit too short, determine how much more you want by tying on some extra line you have so you can be sure you have enough for secondary uses-such as pulling a person back on board; then add that to your overall length.
7/16" is pretty big stuff for a 29, though-the sail won't fly too well in light air with a couple of those hanging off the clew...For sure, ease of grip is a consideration-but go as small as you can while keeping an acceptable diameter for grip-remember that larger stuff move a lot slower through your blocks....
By the way, once you buy some high quality halyards, you might want to use those messenger lines-all or most of the time. Just take the halyards down after you are done sailing-if in salt water, rinse them off then store them down below-good way to protect your investment. They will last a LOT longer. If this is too much work, you can tie the messenger to the shackle end of the halyard and hoist it all the way up..At least this way (provided you have internal halyards), you will get the good stuff out of the sun and elements. When ready to sail again, just pull the "1/2" messenger (and halyard) back down to the deck and off you go.
Sorry if this is a repeat-I know I mentioned this before, but my befuddled brain can't recall whether it was in a private or general message.
Cheers,
S