While such a change is, from the standpoint of architectural purity, absolutely the correct thing to do, I fear that in practice it may lead to confusion, needless duplication of records, and a failure to achieve the intended objective. In other words, it is my opinion that engineering considerations do not weigh in favour of changing the string.
I particularly do not want to see a world where the norm is:
example.com TXT "v=spf1 a mx ptr" example.com TXT "v=spf2 a mx ptr"
Both records would contain the same content, yet senders might feel the need to publish both "just to be on the safe side".
Both records do _not_ contain the same information. The first record has a target of MAIL_FROM; the second has a target of PRA. The fact that the strings "a mx ptr" happen to be the same in both records does not change that fundamental fact. The fact that the same IPv4 dotted quad might appear in an MX record and NS record for example.com doesn't change the fact that the records tell you different things about how that domain's systems are set up. The situation here is parallel; it isn't as obvious because you're using the overloaded TXT record rather than the new RR.
As an aside, this is one of the practical reasons why subtyping the TXT record is a bad idea; it gets very easy to confuse the types, even for folks who have been working with it for a long time.
regards, Ted Hardie