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I am not sure what you mean. The latest update in the 1.6/1.7/2.5 release branch is 2.5.9. All versions of Joomla! 1.6 and 1.7 have known security vulnerabilities, are no longer supported by the Joomla! project and were taken off-line. You can't upgrade to a 1.6/1.7 release because there is no way(*) to get the update files any more.

In any case, I DO NOT recommend staying on Joomla! 1.6/1.7. There is a known high priority security issue which allows a gues to subscribe to your site as a Super Administrator, without your consent. It takes 5 minutes, it requires no skill (we had a non-developer follow the procedure without a problem) and there is no security software which can protect you against it. Joomla! 1.6 and 1.7 is insecure and continuing using it will get you hacked. It's not a question of "if", it's a question of "when".

* Actually, there is. If you are a member of the Joomla! Bug Squad you can log in to JoomlaCode.org and see all unpublished releases of Joomla!. But you will have to do a manual upgrade and, still, you would end up with an insecure system.

Ah, I see what you mean. Doesn't Admin Tools propose to upgrade to Joomla! 2.5.9 from its Upgrade Joomla! page?

Ah, yes, now I remember what's going on (it's been almost six months since that release of Admin Tools became end of life and no longer supported). Back then Admin Tools had to be manually tweaked to select the latest STS or LTS release of Joomla!. Back then, STS was 1.7 and LTS was 2.5. Since people didn't want to upgrade to 2.5 that release was set up to only show STS updates. While they used to be 1.7, right now the STS is 3.0 and this is what it shows you. You can't change that. There is no option to do so in that old, obsolete version of Admin Tools.

In any case, this kind of upgrade is a major change for your site. I would suggest taking a backup of your site, restore it locally and use Joomla!'s own updater to upgrade to 2.5.9. This will work. The only reason Admin Tools was working better than the 1.6/1.7 Joomla! updater had to do with the kind of CPU usage, performance and PHP timeout restrictions you see on most shared / VPS servers. That's also why Joomla! 2.5 onwards includes the Joomla! Update (com_joomlaupdate) component written by yours truly which is nothing more than an adaptation of Admin Tools' Joomla! updater code (with minor changes regarding the source of updates and the download method to match the Joomla! project's guidelines).

When you're using an STS release of Joomla! (1.6, 1.7, 3.0 or the upcoming 3.1, 3.2) you have to accept it as a fact of life that in 6 months at the latest they will not be supported by the Joomla! project and extension developers. STS releases are supposed to be a stepping stone and you are supposed to upgrade to the LTS as soon as possible. I try to support the previous STS for about a month since it becomes end-of-life, unless there is a serious reason not to (e.g. supporting the STS prevents my software from running on the current LTS). This is what happened there. In order to upgrade my software to properly run on 2.5 I had to completely drop 1.7 support.

Regarding your methodology, I think it's the correct approach.

BTW, when upgrading from one J! version to the other it's almost certain that you will need to use the database fix feature. That said, I have never observed any data loss doing that.

Thank you very much, Vince!