Actually, changing from the B/X spell progression to the BECMI spell progression is a huge change, if you take the economics and demographics of your fantasy world as seriously as ACKS assumes you do.
Under ACKS/BX, 7th level clerics can cast Restore Life and Limb. Take a kingdom of 6 million and the default ACKS demographics of leveled characters. That means there are 245 clerics of 7th-14th level who will be capable of casting Restore Life and Limb. There will be a total of 280 castings per day, and based on the formula used to derive costs to cast spells, it will cost 500gp for the spell. To have a single caster of 7th level requires a city of 50,000. T
Under BECMI, it takes a 10th level cleric to cast Restore Life and Limb. In the same kingdom of 6 million, there will now only be 12 clerics who can cast Restore Life and Limb (1 in 500,000), with a total of 13 castings per day. Based on the formula used to derive costs to cast spells, it will cost 2,200gp for the spell. To have a single caster of 10th level requires a city of 500,000 - of which there are probably no more than 1-3 in an entire continent. Since the efficacy of Restore Life and Limb decreases with each day after death, most adventurers will be unable to get to a cleric who can restore life, since their adventures will tend to take place far from Class I markets.
So this one small change means that Restore Life and Limb will be 1/20th as common at best, almost entirely unavailable to adventurers who are actually adventuring, and hideously more expensive. It suggests a game world where the emperors and kings might be restored of life and limb, but almost no one else is.
Then consider: What would happen to adventurers, as a social class, if Restore Life and Limb were 1/20th as common? It’s likely that fewer would dare to become adventurers at all, and many fewer would survive to high level. So the demographics of heroism would probably have to be revised to reduce the number of high-level clerics. If we assume the average 10th level adventurer gets killed just one time between 1st and 10th level, that would mean that 10th level adventurers would be only 1/20th as common!
But that means that 10th level clerics would be even less common! And that would cascade again: Now perhaps Restore Life and Limb is 1/50th as a common. Now 10th level clerics aren’t in 1 in 500,000 - they are 1 in 2.5 million. Now a cleric capable of casting Restore Life and Limb is a prophet who changes the world.
Now, you could overcome this by adopting more favorable demographics for high-level characters. Perhaps in your campaign world, 10th level characters are as common as 7th level characters in the default ACKS system. But that causes lots of other problems - because that means there’s a LOT more treasure in your world, for one thing. Which means that all of the wages and earnings of nobles need to be revised. And on and on.
The economy in ACKS is built on an interlocking system that runs across 23 different worksheets in EXCEL.
You can of course simply hand-wave all of this, and keep Restore Life and Limb at 500gp available at Class III markets. But at some point you end up with awkward questions.