Would it be simple to change the armour from 20 to 40 and change the cost from 20 to 40?

Would it seriously impact the game balance?

It's possible with the Modding Tools shared elsewhere on the forum here

But bear in mind, this is Epic 40K - the rules that the game uses are based on the rules for Epic 40K 1st Edition - indeed they are almost identical in function

So, much as you may love your marines, the fact is they are just as wimpy in the tabletop version at this scale

Part of the joy of Epic scale 40k conflict is seeing whole forces get wiped in a single salvo

I think they have a chance to hit every figure (not just every squad) on the tile they land on, and the ones adjacent to it, so they can do nothing, or wipe several squads out.

It is super simple actually. The steps are described in the mod guide:
download the mod tools
update the UNIT.TEXT
Change "Space Marines" from 20 20 20 / 20 20 20to:
40 40 40 / 40 40 40
OR you may want to make it: 45 45 45 / 30 30 30 or something, so that hits have a chance to hit the mosrt protected parts, but also the head, meaning easier penetration.
You need at least 35 to give them a chance to survive a lasgun/bolter/shotgun hit. Anything below is worthless.

Then launch: UnitToDat.exe and replace the file in your DAT folder.