This is simply the wrong way to go about ranking up in the Grand Companies.
Exchanging GC seals should be reserved for the gear or rewards that the player receives from the Grand Company and not used for ranking up.
There should be "Rank Up" points which are accumulated separately from GC Seals.
I would like to call your attention to the Mercenary Rank system in FFXI:
"The Mercenary Rank system works similarly to the Rank system. With a higher Rank, you are able to buy more items with Imperial Standing Points at an Imperial Gate Guard and the Sanction status will last longer. However, rather than donating crystals to Conquest Overseers and doing nation missions, you must successfully complete Assaults for your promotions.
To earn the next Mercenary Rank you have to accumulate 25 "Rank-Up Points" in Assault Missions"
It is not a good practice to reward players with one currency and then penalize them by taking away that currency. If it is a rank system then there should be a separate method of determining the experience or work completed towards gaining rank rather than taking the seals from the player.
I strongly urge Square Enix to rethink this change.