This is fine. If you want to be strict with yourself just pretend you started working March 2 in the Morning and submit by March 9 same time of the day as a 7DRL entry. Otherwise and if it's going to be different enough than the initial entry, you can start now using it as a base, and submit by March 10 23:59.
Please create a separate page and state that you started from other entry, just for clarity of the reviewers.