The International Cessna^® 170 Association

Does any one know why the shape of the elevators change from A to B? I suspect the flaps had a play in it.

I don't know perhaps it has been discussed in one of the books Cessna development.

The different shape may be disguising a slightly larger area for the elevator but I don't know that the B elevator has more area. It might have been to facilitate the balancing of the B elevator as the B elevator cord is shorter. All of it could have been to improve elevator performance in some way for certification with the different flaps, dihedral and washout in the main wing. And just maybe it was cosmetic but I do think there was more to it than that..

The chapter covering the development of the 172 in William D. Thompsons book Cessna Wings for the World discusses the issue of the need for more elevator power in the 170B. Paragraph reads:
The impressive flying qualities of the Cessna 180 and the need for updating the C-170 led to the design of a new empennage for a planned 1956 C-170C, as early as 1954 a C-170-B had been fitted with a more rectangular planeform tail to increase the aspect ratio and, therefore, the stability and control. In particular, there was a need for more elevator power (with reduced elevator stick force) to make three point, Full flap landings easier at the most forward C.G. position. An updated Continental O-300-A engine capable of producing 155 HP (I wonder where that engine is? my comment) was also installed, but the extra power further aggravated the long-standing oil cooling problem. This attractive C-170C is pictured at the end of this chapter.
My comments:
There is no comment on increased elevator area or any mention increase stabilizer elevator area in the book that I can locate. There is also no mention of why that the Cessna 170–B has balanced elevators and that the 170 and 170-A did not. However from the comment above from the book it can be surmised that there was a need for more elevator power in the 170-B - “In particular, there was a need for more elevator power (with reduced elevator stick force) to make three point, Full flap landings easier at the most forward C.G. position”.
It has been my observation that flying the Cessna 172A converted to a tail dragger is that the stick forces in landing with full flaps are heavier than those of the 170-A or 170-B, I have never flown a 170 rag wing. One thing I have noticed is that the trim availability on the 172-A is limited to 10 Degrees up where the 170-B trim has 28 Degrees nose up. We used to have a large lead weight at the rear bulkhead of the 172TD to compensate for this heaviness at the full flap landing. We were eventually able to remove this lead and have lightened stick forces at the full flap landing position. I will let you guess how we did it.

Indeed, the elevators were enlarged and reshaped and balanced in order to accommodate the larger pitch angles the semi-Fowler flaps created. W.D. Thompson's writings confirm this.
The one C-model which was used to develop the square tail was converted back to round tailfeathers and sold. Instead of a C-model 170, Cessna produced the 172.