Pikemen are the main army during the medieval age (no SR requirement) so if you have a lot of them you will probably not be able to upgrade them all to Lancers since (I guess) Lancers require the SR Horses.
So you (probably) will have pikes and musket together.
Yes, but someone was saying they upgrade to pikes. I am not quite there yet in my game. I just got to musketmen. I'll let you guys know soon. The thing is you'll never use as many lancers as you have pikemen anyway, so it makes no sense to have pikes up to lancers.
They do upgrade to pikemen. Fine, I'll upgrade my best units to lancers. I have plenty of horses, so if they are required, no big deal.