Well... is there any reason why you don't want to upgrade VMFS ? Do you still have hosts older than ESXi 5.0 accessing these datastores ?
It only takes some seconds to do that (and you probably should have a backup...just in case...but you should have a backup anyway)
Based on your answer i looked into this again. So i can update to version 5 without having to create new datastores.
Is there any impact on the running VMs?
VMFS 3 datastores can be upgraded to VMFS 5 while VMs are running. There shouldn't be any impact; I just did this on Monday during a 5.5->6.5U1 upgrade.
There are some differences between new VMFS 5 datastores and VMFS 5 datastores that have been upgraded from VMFS 3. Best practice is to create new datastores and migrate VMs but that requires available storage space to get all VMFS 5 features.
You can upgrade VMFS from 3 to 5 but I would not recommend that. I would instead create new datastores with VMFS5 so the block size is 1MB and migrate data to these new datastores. Upgrading VMFS maintains the block size that VMFS3 was created in. Basically restricting VMDKs to 2TB in size. VMFS5 allows VMDKs to be 62TB