It sounds like your requirements are to reduce cost, reduce complexity, and simplify operation. The points mentioned above about hardware redundancy are certainly valid and need to be considered. However, if you are set on doing this, you could just keep 1-2 of the old blades as cold spares in case you enounter a hardware issue. Since you have 7 of these old blades, you would be able to have plenty of offline cold spare parts if needed.
Just another thought if these are not super-critical and you can afford some downtime.
@jonathanlackman