Why am I being asked to pay for iPhoto when I already have it? I just upgraded from Snow Leopard to Mavericks and when I run 'check for updates' in iPhoto it says there is an upgrade available but that I need to pay for it (this was not the case with the GarageBand update)

Why is this happening and how do I get round it?
Did you change your Apple ID? If you bought iPhoto in [apple ID 1] and you are using [apple ID 2] now, may be it will charge you.