Q: Photos asking for Free Space to Migrate iPhoto Library

You will need some extra space to migrate the library.  When your iPhoto library is migrated, Photos will need working space during the migration and it will need space for thumbnails and other working copies, even if all original image files and previews will be shared:  Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Any tips? I only have 9gb of free space on this Mac.

With only 9GB of free storage you are already at the limit and need to free space before you proceed. 

Depending on the vintage of your MacBook/Pro and its port, speed, and data rate, an external

hard drive of either ultra-fast rotational speed or SSD may work for a on-site external photo

library archive. If your computer has Thunderbolt ports, an adapter may allow one to utilize an

external enclosure with fast data flow rate, so the internal drive may not need to have the

library stored on its small/ish internal hard drive. A quality enclosure would need research to

locate and a suitable storage drive matched.

The external enclosure probably would need to be self-powered, and perhaps have more than

one hard drive or other storage device within it. Some of these enclosures offer a variety of

port types and to match a suitable storage drive with the enclosure, and to port speed, is an

important aspect, especially if you should ever need to use a partition on the external drive to

run the computer from a clone of the OS X. (Another unit should be used for backups of OSX

and not just Time Machine;  a cloned OS X can save time from recovery/update by download.)

A different method of handling larger libraries is a matter of budget and research, then purchase.

In the meantime, until you can see what OWC or other reliable vendors of quality gear have, you

may have to settle for devices that can hold backup copies and duplicates of them. Not sure of

ideas to consider multiple storage drive external RAID? enclosures that make duplicates, would do.

Anyway, I see the situation of upgrade to a later OS X to also invite other major revisions, too.

Hardware usually follows an operating system upgrade, in one form or another.

In any event...

Good luck & happy computing!

I didn't anticipate this issue when I upgraded to Yosemite. Right now, I don't have access to my photos. I hope someone here can help. I've searched the internet quite a bit, but having trouble finding any solution.

Are you planning to use iCloud Photo Library?  iCloud Photo Library FAQ

If you invest into iCloud Photo Library you could keep the originals in iCloud and only keep optimised smaller versions of your photos locally. But that will require monthly fees for a storage upgrade, but you could access your photos online without external drives connected.

if you prefer to migrate the libraries to an external drive I'd go for a small portable one, that is no hardship to carry around with your MacBook pro.

In any case, to migrate the library to Photos you first need to move it temporarily to an external drive, formatted MacOS Extended (Journaled) and locally mounted, as described here for Aperture:  Format external drives to Mac OS Extended before using with Aperture

Photos will not create a migrated copy on an external drive, if it is formatted differently.

• Copy the iphoto Library to that external drive.
• Drag the iPhoto Library to the Photos icon in the Dock.
• Wait for the upgrade to finish.

After the migrated copy has been created, you can use it from the drive or upload it to iCloud by enabling iCloud Photo Library.

I only have 9gb of free space on this Mac

It's strongly recommended one have a minimum of 10-15 GB of free space on the boot drive to facilitate optimal system and application performance.  As léonie has already pointed out your first order of business should be to free up additional space before proceeding.