Are there any standard techniques for comparing signatures to determine the likelihood that they are or are not by the same person?
Yes, but they are based on comparing multiple examples. They form part of a body of techniques known as Handwriting and Signature Analysis used by forensic specialists (not to be confused with the pseudo-scientific techniques for determining personality based on handwriting, known as graphology). In the case of signatures:
"... detailed microscopic comparative examinations are made of a set of known ("specimen") signatures and one or more questioned signatures, the writer of which is in doubt."
Those techniques would not be applicable in this case. Here we have only one specimen signature, and the pair are separated by half-a-lifetime!
Many things can cause changes to our signature over time. One very famous example being the signature of Guy Fawkes before and after he was subjected to "the gentler Tortures":
While there is nothing to suggest anything so extreme in Daniels case, it must be noted that we know nothing of changes in Daniels health over he intervening 42 years. To give just one example, osteo- or rheumatoid-arthritis in his hands might have made it more difficult to hold a pen (in modern populations, between 10% and 20% of adults exhibit symptomatic osteo-arthritis by age 60. The percentage increases with age).
Conditions like these, which have a gradual onset, would have resulted in changes in his writing style, and so also to his signature.
As a result, none of the standard techniques for comparing signatures are really appropriate in this case. You would need (preferably multiple) examples of his signature from the intervening period in order to make a meaningful comparison between them.