What changes if Russia goes to war officially?
Russia has already opened up the enlistment age. Hardly sounds like they have ample replacements. We have video evidence of t-62 in use by the Russians. Doubt they dusted those off because while they had plenty of t-80s in reserve. The VDV went from an airborne unit getting dropped behind enemy lines to support troops. How is the Moskva 2 coming?
Both sides are taking losses. Both sides cant 100% replace those losses like for like. I have no idea which side can sustain it better, but I bet both sides are lying. I am not willing to accept one propaganda over the other because they are BOTH propaganda, and both worth the same amount of trust. Truth has ALWAYS been the first casualty of war. It's why predictions are useless. It's easier to sustain/accept losses as the defender. You have more to lose, and usually a more willing population.
As far as what could change that would effect the war, anything. Political unrest at home is always an issue in wartime. Remember russia was arresting protestors early on. Probably still going on. All the fires behind the lines dont speak to a happy home front. Putin actually gets sick. Ukraine cuts that bridge to/from Crimea. Putin has another conflict pop up.
Even without an outside change, militaries wear down. Troops get war weary. Someone loses an important leader/general. Some stockpile runs out. Heck you lose one battle, take a few too many casualties in one place, and that's a momentum shift. Even losses can be strategic wins if you inflict more losses than the enemy can sustain. Heck season changes have been known to change the direction of conflict on the steppes a time or two.
Again just because you are winning doesnt mean you will always be winning. And what winning looks like can change.