Personally I've never seen the planning date parameter have any impact. It seems to always use the system date. However if you are putting a very old date it there and it works SAP wont have an issue with any requirements with a start date after that date.
There is also a parameter in configuration to "allow start date in the past". You should check if you have that set.
Also with regards to this "when we run this it just adds new purchase reqs to fullfil the new needs vs deleting the old one and creating 1 larger purchase req":
This isn't what happens in my experience. MRP generates a requirement for each requirement date even if they have the same date. If have a 7 day planned delivery time and you need 10 in 2 days and 10 in 7 days you will get two requisitions/planned orders with the same dates. One of them will have an exception telling you to reschedule
Regards
Robyn