Just nitpicking: You could justify increasing a regular shield that way, but not a rote. If you do anything to change a rote, it isn't a rote anymore, and you'd best get rolling. 
You may not increase the number of shifts of the original spell, but I would make a case that a caster can use an action to Prolong a Rote Spell, and in that process, tag an Aspect to increase the Discipline check required to control the Prolonging, making it easier to Prolong but not necessarily a tougher Shield.
