Res changes never hurt. The only thing is if the problem is in the net pot (like some bad disease in a Rockwool cube) then you wont have fixed it. In DWC I always recommend when in doubt...dump it out.
I would use up the hydroguard because it is just going to go to waste. Many people love it...it is based on good stuff. These bacteria only need to be added once every 2 weeks or so. But if you are into using them then it is a good idea to do it at each new res. unless you are dumping weekly. I go not less than a month per res and if I pay attention I will go all the way through veg on one res. I will dump when I want to change the base. But I do some fancy things right before to get the puberty thing to happen fast so I can sex them immediately. So I like to dump after all that jazz. You don't need to do any of that.
Here is a little trick I do when adding either Bacteria of Fungus...I would recommend taking the hydroguard and diluting the stuff a little into either res water or top off water and then introducing it to the res through the net pots. So for example you get a 2 cup measuring cup. dip out some res water. In your case add 1/5 of the amount of hydroguard you plan to add to the res to the cup of res water. Then dump that through one of the pots. Continue on all 5 pots and then the bacteria and fungi will have a good chance at taking hold in the top roots.
I would use up the hydroguard because it is just going to go to waste. Many people love it...it is based on good stuff. These bacteria only need to be added once every 2 weeks or so. But if you are into using them then it is a good idea to do it at each new res. unless you are dumping weekly. I go not less than a month per res and if I pay attention I will go all the way through veg on one res. I will dump when I want to change the base. But I do some fancy things right before to get the puberty thing to happen fast so I can sex them immediately. So I like to dump after all that jazz. You don't need to do any of that.
Here is a little trick I do when adding either Bacteria of Fungus...I would recommend taking the hydroguard and diluting the stuff a little into either res water or top off water and then introducing it to the res through the net pots. So for example you get a 2 cup measuring cup. dip out some res water. In your case add 1/5 of the amount of hydroguard you plan to add to the res to the cup of res water. Then dump that through one of the pots. Continue on all 5 pots and then the bacteria and fungi will have a good chance at taking hold in the top roots.