The IIa can do 35mm up to 6x9. The advantage it has over the IIc is that the negative carrier is slightly larger than that of the IIc. Cameras give us different sizes negatives. Normally that's okay but I have some negatives from an old folder that don't fit well in the carrier of the IIc. For those I filed out one of my IIc's neg carriers.
The early IIa came with only one lens, the later with two. I am not sure how practical it would be to use the early version for 35mm. If you have space, keep the Ic next to it as each enlarger gives you extra possibilities over the other.
The light system of the IIc is better: the IIa uses tiny springs to keep it in place and centered. These springs are often missing, which causes uneven lighting.
A little warning, the IIa can be significantly older than the IIc and parts will be more difficult to find. The later IIc's Elmar 100mm and the Focotar 100mm were improved significantly.
Finally, do not expect much from the Leitz manuals in terms of operation. These mostly list the parts for these enlargers.
And, I would not seek to replace your Ic. In fact, the IIc / IIa are great in combination with a Ic. Because for 35mm the large condensers of the IIa / IIc can solve difficulties when it comes to totally even prints.