The fact is, some of the previous ranges were based on best estimates given the actual replacements found, and our understanding of production process. Given the vast knowledge of the people up top who placed the ranges, they are very likely accurate.
Note that the Journey series has had a number of corrective changes that go retroactively to earlier in the series. This is due to our far increased knowledge of the production process. Replacements that were found STILL REMAIN in the range. The range perimeters were adjusted to properly reflect the proper information. Now that this has been done, "changes" (corrections) need not be feared, since they are done already.
Regardless, older Journey replacement notes, and Bird replacement notes likely already have the market suply established and thus are priced accordingly. For example, the sheet replacement HPA is very rare --> we already have fair market pricing for it due to supply and demand.
That is an important concept: Above the SNR/Sheet issue, is the fact that there just might be a super small supply. This gets reflected in the price.
Changing a prior established range X from a sheet to a SNR should't change anything because the amount of replacements still remains the same. And if the market indicates a high rarity, then it will already be reflected in the price. (HPA for example)
In saying that, we KNOW that SNRs are rare - but we already "know that" by virtue of them being SNRs vs Sheet replacements. That accounts for the difference in the price.
Hope this helps.
Canadian Paper Money
