A good Land Contract document addresses a secondary sale within its terms (all of Standard Legal’s Land Contracts do!). If the existing Land Contract spells out the terms for any sale by the Land Contract buyer, those terms should be followed expressly.
If the Land Contract does NOT address a resale, then the original buyer can assign the land contract (in writing) to the new buyer under the existing contract. The new buyer would pay the original buyer a set dollar amount for the assignment of the contract, then the new buyer would simply continue making payments to the seller as outlined in the existing Land Contract document, as well as be bound by all other terms.