Hi Fara,
This is a great question. The short answer is the dried leaves are still useful for nersvous system and digestive support, they just aren’t as effective for HSV. Just give them a smell, this will help to know their strength. When drying make sure there is little to no heat involved and just light airflow to reduce the evaporation of those ‘light’ essential oils.
Freezing them is a very interesting idea. I imagine that could work, I’m just not sure how well they would freeze ( it might give them frost bite or something?) as they are picked. What I do is make the lemon balm into oils, tinctures etc. and I make a few lemon balm cubes for freezing by blending the lemon balm with a bit of water, so it’s like a little lemon balm ‘shot’ This is nice to add to water or tea in the future but will be no good for making other preparations as the water is already added. Directly freezing the leaves could be a great experiment to try as part of your preservation of harvest this year and then if it worked well, do more next year.
Warmly,
Benna