Well in the case of my card the stock fans are attached to a plastic shroud which screws into the metal heatsink with four small screws, two on each side. I'm confident that I could restore it to it's original state if keep hold of the screws. Card is a Gigabyte Windforce two fan 1070.
I'll probably sell it on with the news fans attached and an explanation in the future.
My case is horizontal, under the bed, so sagging won't be a problem for me.
Carried out the mod on the EVGA 1070 SC. In fact three mods in total lol. Unlike the Windforce, this card had hex-like screws for a tiny allen key as well as normal screws. Lacked the right allen key but managed to remove them very patiently with pliers.
Fans used: 2 x Phanteks MP120 PWM
GPU fan adapter: EKWB GPU fan adapter
PWM splitter: a Noctua I had laying around.
TIM: Thermal Grizzly Kryonaut
Other: Zipties, velcro and some wire.
Mod 1: Fans attached to card's heatsink with zipties. Brought temps down by about 7C and slightly less noisy at 55% RPM. Even quieter in comparison if RPM pushed up.
Mod 2: To avoid sagging, I experimented with attaching the fans to the PCI bracket below the card. Now the fans sagged so used some wire attached to the fans at the opposite end, to keep them level. Was interesting and I was glad to not be pulling on the heatsink anymore, however temps worsened by 5C because there was a 1.5cm gap between the fans and the heatsink, no shroud to funnel that air for static pressure, and being 120mm fans on a 80/92mm heatsink, even more air was going to waste with that gap.
Mod 3: Had a look at the vents on the card's own PCI bracket. I'd run out of zipties at this point lol but had some velcro which I trimmed to fit through the fan screwholes and the said vents. Got the fans nicely pressed up against the heatsink in this fashion, and used wires at the opposite end again to keep them level and taught. The outcome was better than the first two - fans right against the heatsink and temps back to Mod 1 levels + fans now act as anti-sag support for the card.
Had no real need to do it but it was fun to mess around with it and see a little improvement at the same time. Now running a fan curve of 45C = 35% RPM, 50C = 40% RPM etc. Games aren't taking it over 55C so at 45% it's very quiet. 1.0V on the core @ 2025MHz.