i believe if you modify a tech, the changes you make will effect everything
after you make the change, but whatever the effects were before would remain in place. I'm not 100% sure however. I only say this because I've modded naval doctrine techs before and found that existing naval units were not effected by the change I made, but new units were.
Also, given there are far fewer participants in the modding sector than other areas of the forum, i would suggest waiting more than 6 hours for a reply.

I know from experience it can be excruciating waiting for help, but patience has always paid off for me in the past.
Anyway, hope I helped. Check HOI2Wiki, they might have something on the topic.