Hot Water Line Restriction

My hot water line to our upstairs bath is restricted somewhere. Both the sink and tub hot water runs slow. I found the supply line in the basement that runs up to the bath. From the hot water heater, there is a supply line that runs to it and splits to the washer supply. The washer supply runs fine so it's from that point to the upstairs. It's the old iron pipe also. I want to cut the horizontal pipe in two and then snake the verticle line in an attempt to free it. The iron pipe is 7/8" thick (called one inch?) My question: Can I put a copper adapter to 90 iron elbow and then sweat copper on it? I would end up replacing about three feet of horizontel iron pipe with copper pipe, valve, fittings. I seem to remember dissimiliar metals cause problems but don't want to mess with iron pipe and don't want to replace the whole house with copper. Thanks.

You would be better to sweat the threaded fitting on the copper then screwing onto the iron pipe. This would keep from degrading the pipe joint compound or tape and keep from causing the metal at the joint to swell from the heat which could result in a leak. Make your final sweats on fittings away from the threaded fittings. That 7/8 stuff is probably 3/4" galvanized pipe. If the iron pipe (galvanized pipe?) has gooky junk growing in it or obstructing it then you'd be doing yourself a favor to replace all of it that you can get to.