You can't just change the chainrings on your existing crank since the bolt circle on your current crank (135 mm) won't take a chainring smaller than 39T, but you seem to know this.

If you buy a complete Campy compact crank it may be able to use the current bottom bracket depending on what you now have. Otherwise both a new crank and bottom bracket will be needed but that's all.

If you do have to change the bottom bracket, you aren't limited to a Campy crank anymore so any make of crank and matching bottom bracket can be used, probably at lower cost, unless "contaminating" your bike with non-Campy parts is unacceptable.

You will have to lower the front derailleur to obtain the proper clearance over the new large chainring but the current front derailleur and shifter should work fine.