Walking in February and March is possible, but you gamble with the weather and a small number of people around. In terms of the Aubrac Plateau, the Gendarmerie now block the route and make you take the road if there is snow and the weather is bad, so you can take comfort in that. We have seen pilgrims in January who pass the plateau with no problems, with pilgrims in April being diverted because of snow. You never know.
I think in February the numbers of gite open will be less, so your choices could be more limited. 1st April is the traditional opening date for most gite.
On the way everything is possible but it is up to the gods what weather you get.