Installing Debian 10, upgrade to stable later?

Actually you won't even have to upgrade or do anything, it will happen on its own: buster will become stable.

If you're using the code name buster rather than the archive name testing for the repositories, then there's nothing to do. If the installation used testing you can change it to buster yourself (in /etc/apt/sources.list*).

When the stable "pointer" will move from stretch (the codename for Debian 9) to buster (codename for future Debian 10), buster will become the stable version of Debian, without any intervention from your side. You'll only notice it because the rate of upgrades will probably slow down.

Note: of course, as OP and wurtel said, usual upgrades have still to be done regularly (for example with apt update + apt upgrade) for anything to happen.

You may need to run apt update and then verify that you want to move from "testing buster" to "stable buster". For example:

$ sudo apt update
...
E: Repository 'http://security.debian.org/debian-security buster/updates InRelease' changed its 'Suite' value from 'testing' to 'stable'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Do you want to accept these changes and continue updating from this repository? [y/N] y

Using aptitude or apt-get update may not transition without an error. You might get an error that looks like:

$ apt-get update
E: Repository 'http://security.debian.org/debian-security buster/updates InRelease' changed its 'Suite' value from 'testing' to 'stable'