I'm just trying to upgrade firing button and 510 top cap.
Yes it would be safe, but as Cacique said,
only if you know what you're doing with all the aspects involved in this modification. You need to know how to solder well, have the mechanical skills required to do the mechanical parts of the job and the electrical knowledge required so you don't mess things up from the electrical side. You have two batteries in there and if things go really wrong that's a lot of power that could be unleashed in a small space and do a lot of damage to you.
OTOH, I have a Sigelei 150 and though I would be comfortable doing this mod I see no reason to. Both the switch and the 510 connector are really pretty good IMO, at least after a couple of months of use.
Whatever you do, be careful and be safe
