That's a very good question. I love it when people put some thought into major tank changes
My answer would be to go ahead and plant it all at once. The only caveat being that you should make sure that the your tank is ready to support the plants you have in mind (i.e. lighting requirements, water parameters, substrate, etc.).
The proper plants will only help your tank rather than hinder it. As long as your tank is prepared to support them. The only drawback would be if your tank parameters were not adequate to support the plants and they began to deteriorate, causing large amounts of waste to enter your water.
I guess what I'm saying is, yes, do it all at once, if your tank is ready for it and meets the needs of the plants (not really hard to determine).