I have done a complete substrate swap on my 75g before without and issues. I only tested the water a couple times afterwards, but seen no spikes and this was before I kept plants. I think with plants you should be fine as long as you keep the existing plants, rocks, decor, etc.....
On a side not I found that taking an old garden hose, cutting off the ends and just siphoning out the old substrate works the best and is the easiest method. I ran the hose out the window into a rubbermaid tub where the substrate would fall and settle into the tub and the water would just overflow the top onto the ground. I did this all with my fish in there and was no problems at all. Sure beats scooping it all out with something for sure.
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