Hi,
it is not possible to upgrade from tier x to y
you have to remove the panel and only after you can place the next tier

Hi, any news about this request ?
It was very cool if we can upgrade solarpanel/accumulator from vanilla to MK1 to ... MK8 :)

I have published a 'redux' version that this issue and more and will soonish make an attempt at rebalancing items.

Please give it a go as I haven't fully tested this part. Fast replace work and the upgrade planner accepts my changes but didn't test if the robots do their job.

Link to my version: https://mods.factorio.com/mod/Factorio_Power_RE