The steps I outline here and and how closely you follow them depend on your level of aversion to downtime and disruption for your users.

You have an active userbase and forum so this is what I would do.
1. COMMUNICATION -
Let your users know that you are planning to make a major change to system and let them know what the known impact is going to be.

For instance, bookmarks to threads will no longer be valid, similarly emails that notify you of PMS and POSTS. (probably not a big deal, but part of good manners)

2. SET A TIME
(You should by now have an idea of how busy your site is pick a time to make your switch and announce the final date/moment a day in advance)

 8)3. BACKUP the entire site as it currently is.
There is a BACKUP componnent here that will backup all the files and allow you have the resulting ZIP,TAR or GZ file emailed to you. I've never been able to have it back up the database though, so you will have to backup the database file by file with PHPmyAdmin.

 (FOR THE EXTRA PARANOID/RISK AVERSE)
Setup a seperate site and reinstall everything there, (you will have to change the instances of SPECIFIC PATHS to make it work)
Make sure the restore works.

ERRORS?? DUPLICATE DATA??? AAARGH!!??

(Here's where retroactivity comes in! You should have created the backup with the DROP TABLES OPTION )

THAT IS WHY YOU MUST ALWAYS TEST YOUR BACKUP SCENARIOS!!


Next, setup  the test environment with TinyPortal and install SMF standalone
Use phpMyAdmin to upload the SQL dumps you made earlier.


I've done it a bunch of times. I recommend the JoomlaStarterkit to create a local version of the Apache environment on your local PC.

PM a contact number and I could walk you through it over a weekend or weeknight..

Good Luck!