Well, answering my own question - it does indeed appear the ranged promotions are not useless and convert to melee strength dmg after upgrading to Calvary.
On the icons it will still say ranged accuracy 1/2/3 promos, but when you actually attack a unit and hover over it, the displayed strength calculations and damage computed will take your ranged accuracy 1/2/3 promos into account.
For example - my keshik with open terrain ranged accuracy 3 and himeji castle lined up in flank position with another keshik shows I'm getting the total strength bonuses of: +45% open terrain, +15% friendly territory, and +15% flanking. The total strength calculated looks to take all these bonuses into account.
I'm not 100% sure whether this is just a visual display thing and that when I actually do melee attack the strength is less. Maybe it's just showing it when I mouse over an enemy unit but not using that ranged strength in attack.
But the strength and approx dmg values shown look right and does take into account the prior ranged promos earned.
Now what's interesting as I further level up is that shock 1 was offered to my keshik with already ranged accuracy 3. I took march instead but in awhile I'm going to take shock 1 and see if I get that stacked on top. E.g. Will my bonus now be 45 + 15 from the ranged accuracy 3 + shock 1.
I can't test right now whether the dmg done is actually more or not becaus all my Keshiks have ranged 3 as upgraded Calvary. What I'd like to do is take a keshik with just ranged 1, upgrade to Calvary, and see if it hits with lower total strength and dmg than a keshik upgraded tp Calvary with ranged 3. The displayed computed values will show one as stronger than the other, but I'd like to actually see by the dmg done to two identical enemy units whether the ranged 3 upgraded keshik actually does hit harder.