As of July 1, 2019, the norm (paragraph 10 of part 1 of article 95 of the Law N 44-FZ) is in force, which allows in some cases to change the essential terms of the contract with a single supplier.
The Ministry of Finance of the Russian Federation explained that in these situations, you can change any significant terms, including the price and term of the contract, by agreement of the parties.
Document: Letter of the Ministry of Finance of the Russian Federation dated 19.12.2019 N 24-03-07 / 99602 (Question: about the right of the customer to change the essential terms of the contract with the sole supplier (contractor, contractor)