To find out if you have enough diskspace you do not need to know the size of your actual LVs but the size of your disks. Issue "lsvg rootvg" and "lsvg -p rootvg" to see how much diskspace you have:
"lsvg rootvg" will tell you the PP size, "lsvg -p rootvg" will tell you how many PPs on all the disks in the rootvg exist.
If you have a mirrored rootvg (this is not shown in the "df -k") you can consider a "alternate disk migration". In short this is: split the mirror of the rootvg, update the one while retaining the other as a backup copy. In case everything works fine remirror the newly installed half, otherwise remirror the withheld half. We have some threads here regarding alternate disk migration, you might want to read them and ask if you have questions regarding this.
Having not much time right now i have to stop here. Maybe someone else can explain the firmware update procedures (preferably in detail, so we could turn that into a FAQ), otherwise i will come back to this later.