I've done extensive research on this and there's no custom setups out there that retain factory functions.
As for changing it out, no need to drain the rad. Just clamp off the hoses so fluid is retained within the lines. Any fluid beyond where you clamped will obviously be lost.
Probably wise to get one of those spill free funnels that you can clamp to the top of the radiator to add a bunch of fluid at once and run the vehicle. This will help get air pockets out.
__________________
'99 Toyota 4Runner LTD - Dual Locked | Geared | Mid-Travel | 35s | Armored