doglover asked ... "How many TURBT procedures have you had? Did it suddenly become high grade after the first one, or was it after a second recurrence?"
I had my first ever TURBT last fall (2018), same time as yours. Two very small, low grade non invasive tumors were removed. The high grade cells were discovered only after a second TURBT that I had this fall.
"So you were given mitomycin when two low grade tumors were removed, and then high grade tumors grew?" Exactly, I was given MMC at the time of first TURBT yet within a year more were found, these being high grade. You asked if a recurrence a year later is a bad sign? Do more usually follow? I believe that a recurrence within a year bumps up the risk level a notch: increasing the chance of recurrence, or stage advancement, or maybe both, or maybe neither (I've read so much about this I'm at the stage of information overload).
"Were they in the same location?" Honestly, I'm unsure.
By the way, when discussing the need for a second TURBT, my urologist said that he too occasionally will perform a biopsy (essentially a TURBT) in his office, conditions permitting. He told me that some folks would just rather bite the bullet and get it over with than deal with the hassle of day surgery. I'm not one of "those folks."
So far it appears that our paths have been similar. I hope the difference will be in the outcome of your second TURBT: the cancer that has revisited you is deemed once again, low grade.
As Sara Anne asked, please let us know what you decide and what the pathology report says.