It actually isn't necessary to do either to restructure a contract.
Take Aaron Donald for example. He has 4 years left on his contract and his salary for 2021 is $19.892M, which is already fully guaranteed.
The Rams can convert $16M of his 2021 salary to a signing bonus and get $12M in cap relief in 2021 without extending his contract and without giving him more guaranteed money.
(The math: $16M salary comes off the cap in 2021 as AD's salary is reduced to $3.892M. Then the $16M signing bonus is prorated across all 4 years at $4M per year. So $4M goes back on the 2021 cap for a net savings of $12M, and $4M gets added to AD's cap hit in 2022, 2023, and 2024.)
AlbaNY_Ram
