Let's say that a magus has attuned his talisman a few years ago using his highest Technique and Form combination at the time (say CrVi). Now, after years of study, his new highest combination is different (CrCo). Can he use the new combination to increase the capacity of his talisman? My assumption would be that he can use the new highest combination in any case. It's always Vim vis and the highest Te+Fo is only there to establish the baseline of the maximum power.
Let's say around the same time he had established a familiar bond using his best combination (CrVi). He now wants to increase the bond strength to take advantage of his improved Arts, but his highest combination is now different (CrCo). Could he change the combination used for the familiar bond? This is less clear, but I might allow a change, provided that the combination is still suitable to the familiar. In this case, Vim was suitable for any magical animal. Switching to Corpus would be possible only if his familiar is somehow suitable for Corpus -- such a caladrius, cat, dog, goose or mandrake (all suggested in RoP:M p.57).
What do you think?