Photo library backup

So my MacBook pro is now running out of hard drive space (only 4 GB left).

The majority of my storage space is filled up with my photos going back to 2005, which I have saved in Photos Library.

I've backed up my Library to an external Hard drive, but what I want to do, is delete my older photos from the Macbook (say pre 2010) to clear up some space, and let me update the software.

Can anyone advise me how I can go about doing this, but also still ensuring that I: -

1. can still access the old photos and that opening the library on the external hard drive, without having any photos deleted that were taken since the backup was done
2. can update the photos library on the external hard drive in years to come without it wiping some of the older photos