putting 8.4 water into a 7.8 tank is not a big enough leap to worry about IMO
First you have to consider how much water you are changing out. If you do a 50% wc
then you would have just a .3 difference in pH overall.
FWIW, I have different tanks with different ph levels, some have aragonite sand putting them in the 8's, while others have acid buffers like alder cones/wood that drops them into the 6's. I've always filled them all the same without any consequence.
Another example of swinging pH is a tank with pressurized co2
. It's not uncommon for a tank to drop an entire ph point in a matter of minutes when the gas comes on every morning.
I'd be more concerned with a big shift in TDS