Would it be technically possible for Apple to make some M based upgrade for current Mac Pro owners in form of logic board upgrade or PCI card? Would it be meaning full besides discussion if Apple would be willing to do that at all?
Possible? ( like could a meteoroid hit the bullseye of the Apple campus) yes. Probable ? No.
Put a separate Mac on a PCI-e card doesn't really have much of any upside for Apple over selling yet another box (e.g. a Mini).
Apple sell a logic board upgrade? .... exactly when in the past 14 years have they done that? That pretty much answers that possibility. Apple primarily isn't in the computer parts selling business. They aren't trying to compete with ASRock , ASUS , MSI , etc. They aren't after providing components for Foobar's built-it with a trusty screwdriver and some thermal paste shops.
From what has been leaked so far about the upcoming M1 SoCs. Jade2C and Jade4C being largely just combos of a modified M1 Max ( Jade) die , then there is not PCI-e provisioning capability for a Mac Pro 2019 logic board. They could put a SoC on it with some x1 PCI-e slots, but it is doubtful a large subset of current (and legacy) Mac Pro owners would be happy with that. Even if Apple converted 3 (or 6) Thunderbolt headers into x4 PCI-e v4 provisioning "headers" that probably wouldn't fly too well with many of them either.
The core issue is that Apple primarily has a laptop skewed die. Even if they tweak it ( add an interchip communication subsystem ) to so come multichip module SoC the baseline is still a laptop die. ( they can crank the core count and boost the CPU and GPU benchmarks, but the general purpose I/O would probably still have their laptop constraints. ).
More likely they are going to do a "new" system with a substantively different form factor that probably shouldn't call a "Mac Pro". ( but likely will to hand wave "close the loop" on the transition). That form factor difference will mean it won't be a "drop in" replacement for the current ( or 2012 ) chassis.
Best case will get a very significantly , but not minimal, logic board with just. 2-3 slots with much lower aggregate bandwidth throughput, but something. (e.g., SSD drive slot cards , Audio/Video capture cards , etc. )
Worse case just a much, much smaller board with no significant (relative to Mac Pro history) general I/O.
P.S. there have been more reports about Apple using multiple die solutions for their TSMC N5 and N3 solutions. Those dies used in that multiple "CPU" chip SoC will extremely likely be constructed to talk to special instances of the some generation. ( not a generic M1 die used for most of the Mac line up and the iPad Pro). That isn't going to get you some kind of "add-in" card solution as the comms are specifically designed for single digit "cm" ( or less) like distances.
Without that a. M1, M1 Pro/Max on another card would essentially just be another Mac. (minus some extremely perverse , non-performant hackery ).