The general trick is to avoid such necessity by keeping your isr's short. Life can be cruel sometimes! Another method is to accept the priority but check for the lower priority isr that you want the higher priority on when you enter the isr. I did this in a 'HC11 as it had no means of changing priority.

With your display you might be able to 'pipeline' the display update - because of the serial shift registers, you can shift your data in whenever you can - you only have to update the output register on the timer tick. Such is the beauty of HC595's.
If your device has a PCA, you can config the output to strobe the output latch of the HC595's thus having the hardware do the update always on time. This would also cause an interrupt that would set a flag to tell the mainline code to shift in new data to the registers. Thus the multiplexing is timed by hardware - removing any problems with interruptions from the serial port or anything else. Job done(tm)