BBowker
Yes, this agreement does apply retroactively to valid visas issued before it came into force, as long as those visas are still valid.
Key details from the agreement:
In Article 2, Paragraph 1, it states:
Foreign nationals and stateless persons who possess a valid visa issued by one of the States (Russia or Belarus) have the right to enter, exit, stay, and transit through the territory of the other State during the validity period of that visa.
At no point does the agreement restrict its application only to visas issued after the agreement came into effect. The key condition is that the visa must be valid and issued by the competent authorities of one of the two countries.
This is my interpretation, however I recommend that you also ask at the Consulate.
https://www.mfa.gov.by/kcfinder/upload/files/Press_Service/agreement_Russia.pdf