Aluminum to Copper run
Possible and definitely use AlumiConn and follow the installation instructions. I got a torque screwdriver to pigtail my house (wiha 7.5 to 20 in.lb), but there are alternate instructions that don't specify torque.

You could just pigtail and not replace the aluminum conductor, but you would still want to pigtail the receptacles. The connections at the switches are reportedly less prone to failure, but I do those too and use the two-port Alumiconn connectors. I also use the 2-port on terminal receptacles.

When I am torquing an Alumiconn to 15 in.lb I find it helpful to put the connector in a Crescent wrench. It's easier and safer. I put the copper pigtails in ahead of time so I don't have to change the setting on the screwdriver while installing.

If I am approaching box fill, I use a twist on connector for the grounds, but I put on Penetrox A joint compound. Thirty years ago I pigtailed the receptacles and switches with twist on connectors and NoAlOx (an inferior antioxidant to Penetrox A) and I have had zero failures, but I am slowly going back and re-pigtailing with Alumiconn connectors. I think these are the only DIY connectors for connecting #12 or #10 aluminum to copper in branch circuits which are recommended by Jesse Aronstein http://www.kinginnovation.com/pdfs/alumiconn-docs.pdf