Just saying...
from my point of view I think that not being able to adjust entries from prior to your last fiscal year is a very good limitation in the program. By the time you are locked out of the second prior year records you should have already filed your income tax returns and any changes if not processed properly could have disastrous results with the CRA when filing the next year. If the accountant is making the suggested changes they should understand Sage 50 CDN sufficiently to know that the changes have to be made no earlier than the beginning of the last fiscal period.
I have seen many files in other programs that do allow changes going back a number of years where they were not recorded and reported correctly. The person making the change was not sufficiently knowledgeable but felt they had to 'correct' an earlier entry and never thought to mention it to the person completing their year end work. The cost was large as the accountant tried to figure out why the current year starting numbers were not the same as the closing numbers from the last reporting period. Going back three to five years takes a lot of time.
Just my thoughts...

Seaview Glass said:

but trying to contact them for information

I've found Sage support to be comparatively good, except that they often confuse 'work-around' with 'fix'.   Just because a broken fencepost, some barb wire, and a couple tarp straps kept the combine going until we could finish the field, doesn't mean we shouldn't order parts and plan to do a proper repair.

Seaview Glass said:

disappearing download links

Sage's 'that's old, we don't support it' attitude to maintaining automatic download links and downloads comes across as a kind of contempt. 

Seaview Glass said:

changing features every year that limit your options

My pickup truck is a few years old now, and the people that made it now make better ones with more features, I suppose because they want me to upgrade to the new one, and they are aware that they don't make the only pickup truck on the market.

The Simply Accounting website used to publish a 'brag list' of new features they had added.  

https://web.archive.org/web/20020806174126/http://www.simplyaccounting.com/ce/products/new_features.asp

For me, there's been nothing significant added to Sage 50 since 2011, they won't fix bugs that don't cause a crash dump.  The Daily Business Manager Performance tab displayed wrong, mystery numbers, so they chopped it out rather than fix it.   Sage won't even add a column to a report. 

But it's now a rare day that the program crashes, and that's also worth a lot.

Seaview Glass said:

not being able to change data that is before your last fiscal year have made me feel like my head's going to explode!!

In a perfect world, entries are always made, and correctly entered, by the end of the following fiscal year.   Unfortunately, the timeline in the real world is short - there's never more than six months from the prior year's tax filing deadline to the day that we need to open a new fiscal year, and close the one just filed.  And that's if everything goes to plan.