I absolutely still try to upgrade. I don't think that he is capable of being a 20+ minute per night shutdown LD for a contender this year. Even if he can do that at times, I don't think he can sustain it for 2 months vs the 1st lines of Minnesota, Colorado, Vegas, Calgary, Edmonton, and whoever comes out of the East. I don't see a playoff path where you don't go through at least 2 of those teams in the West and then play a damn good team from the East.
But let's say that he can sustain it and we could somehow know that right now. We are still allowing a lot of shots and chances. The D could still use improvement even if this version of Mikkola is guaranteed to remain. Bringing in a #1LD allows you to give Krug-Faulk even more favorable usage to dominate even more and it allows you to scale back some of the difficulty of Mikkola-Parayko's usage. Or you can split up Krug-Faulk, give Krug ludicrously favorable usage and give Faulk some tougher minutes with the new guy. Getting a guy who can eat into some of Krug/Mikkola's minutes and ease their burden would absolutely be a huge help. I also really like the idea of rotating Scandella/Bortz in and out of the lineup to keep both fresh (or running 7 D with some frequency) if we don't wind up moving Scandella to make it work.
I'm not willing to sell the farm for a pure rental under any circumstances. I want a 2+ year solution if we are giving up prime assets. Even if Mikkola is a legit #4 D man from here on out, the D would still dramatically improve if we brought in a legit #1/2/3 D man for the left side. It would allow us to better shelter Krug and it would make it possible to move Scandella's salary in the summer if he isn't moved in-season. Mikkola at his absolute best makes the blue line plausibly-competent-enough. There is still lots of room to upgrade.