Several companies (Wilson, McCormick) sell 8-round conversion kits with spring and follower. You must test thoroughly, they are kind of walking on the edge of what can be done with a standard 1911 magazine body.
I have only done this on a Wilson magazine. If you are using a Wilson 7 round magazine, you can replace the follower with the 8 round type. There is no 8th round witness hole on the 7 round mags, but otherwise, it works fine.
In the Wilson 7 to 8 rd conversion kits I've installed in standard length mag tubes, the big difference was the spring. The 8 rd. mag spring was thinner wire and had less coils so it didn't take up as much space when stacked at the bottom of a full mag, thereby allowing you to increase capacity by an extra round from 7 to 8.
The easiest way to convert a standard 7 rd. mag to 8 rds. is to just buy a conversion kit with new spring and follower. If you have a Wilson mag, you can try just getting the 8 rd. follower from them as mentioned above.
I would recommend against it. Not worth it IMO. I went the other way due to magazine failures with CMC. No help from them so I replaced the followers and springs with Tripp and fixed the problem, but still left me with a sour taste for 8rd "Improvements". I am not talking about 1-2 mags. I am talking 7 out of 7 Chip McCormick mags. 2 8 rounders, and 5 10 rounders.
My opinion, take it for what it costs, get good mags like Checkmate or Tripp, or Wilson 7 rd, or get extended mags to make the extra capacity, don't do it with followers and springs.