I'll be referencing the "Minutes of proceedings of the Colonial Conference, 1907" throughout (600+ pg. PDF). The page numbers refer to the ones printed on the page instead of any software page number.
It seems that @owjburnham's comment is essentially correct, it is mainly a shift in terminology. It came from a desire to further distinguish self-governing from non-self-governing colonies (or "Crown Colonies" as the official term seems to have been). As such, "Dominion" came to be (re)defined as "self-governing colony."
During the 1907 Colonial Conference, Prime Minister Sir Joseph Ward of New Zealand opined the following [pg. 30-31]:
I think the term "Colony," so far as our countries are concerned, ought to cease, and that that term ought to apply to the Crown Colonies purely, and that those of us who are not at present known as Dominions or Commonwealths, should be known as States of the Empire, or some other expressive word, so as to make a distinction as between the Crown Colonies and the self-governing
He also stated the following [pg. 48]:
I assume that in this resolution New Zealand, now known by the term "Colony," will be included in the expression "Dominion," which I think it ought to be.
Awkwardly, this was right at the end of the day and no one reacted to this statement as the conference adjourned.
As the participants were deciding upon the structure and participants of subsequent conferences, there is a lengthy discussion of the exact term to be used to refer to the self-governing colonies [pg. 78-90]. Near its conclusion, the chairman states the following [pg. 89]:
We agreed [...] that instead of the word "Colonies" we should use the word "Dominions;" but is it sufficiently defined if we use the word "Dominions" alone throughout? [...] I would suggest that we might take what is really the official term "the Dominions beyond the seas" in the first place where it occurs [...] and any other reference to it in the course of the Resolution might very well be "Dominions." That would make it absolutely clear what we mean by the expression in the first place.
Thus, the conference opted for an implicit rather than explicit definition of "Dominion". The designation of Newfoundland as a Dominion in 1907 is just a reconciliation of the fact it was a self-governing colony and the new understanding that the word "Dominion" was to mean roughly that. There was no effective change of status. An explicit definition of "Dominion" would not come until the Balfour Declaration of 1926.
Ironically, Newfoundland played no role in deciding the term to be used for itself as its Prime Minister only arrived on Day 4 of the conference [pg. 87].