My parents are US permanent residents (green card), currently living outside of US with re-entry permit. But they're considering abandoning their green card if it's possible to obtain visitor visa in its place. Do they need to apply for visitor visa separately or is there adjustment that will guarantee them the visa while abandoning the green card?
I know that persons who have officially abandoned their LPR status (using Form I-407) can participate in the Visa Waiver Program. They should however take a copy of the processed Form I-407 with them so they can show airline staff that they have legally abandoned their LPR status. I assume that they can also apply for all types of visas, but I don't believe there is a way to adjust their LPR status into a visa category.
Granting a visa is however always at the discretion of the embassy staff, and entry into the US with a visa is always at the discretion of the CBP officer. Nothing can guarantee a visa and no visa guarantees entry into the US.