Capital budgets and maintenance budgets are very different, sometimes owners do what seems odd or illogical to us but it meets their budget goals and limitations.
In my world, replacing sheets is not that unusual, I'm replacing rings 5, 6 and 7 of a 7 ring tank now. It's easy really - cut out a 20 or 30 foot section creating a window in the shell ring, lock in a new sheet in place, weld one vert and then the bottom round seam, cut out the next section, lock in a new sheet in place, weld one vert and the round seam, proceed around the tank.
Ultimately you are left with the top portion of the tank resting on and held in place on the lower portion with fit-up gear, or stitch welds, C-clips, vertical stantions, etc. It might sound unstable but really, it's not - or at least that has been our experience - and we are very careful to monitor weather conditions, etc.
A tank mostly could not care less if a window is cut out and if you place the new sheet quickly there is little risk of a storm deforming the tank though there is some risk of the weight above deforming the tank at the open corner depending on the tank size, shel lthickness, etc.
Depending on the extent and duration of the repair, the likelhood of seismic or wind events, we might weld say W12 x 45 posts floor to top angle to maintain roundness, stability, etc.
I'm also fortunate to be working with very very experienced tankies...
