yes you would need to convert them all.. When I did it a year and a half ago I converted 600+ and it took a longtime (was before I upgraded to a faster PC). Your best bet is to maybe hire a few to convert some for you unless you have a ton of PCs you could use..
I'll do 1,000... for a price
Easy HTML5 Video : HTML 5 Video Converter
that is what I use and it's simple but takes time.