Would 5.8 SOHO be upgradeable to 5.9 SOHO by using the package manager (assuming a separate /home partiton) ?
I think the answer is no. There is a hole new toolchain, probaly slapt-get would need to download the full iso anyway. You can keep your home partition, though. You should just reinstall and mount your old /home partition. I think is not a big deal, the install should be really easy in general.
Also, regarding the first reply, isn't 5.8 SOHO based on Slackware 11 ?
Yes, you can use the Slackware 11 packages on 5.8 releases.