url changing

[caves.ru]

Decided to upgrade to 5.1.X but got information about url changing.
Is it true? Can I still access to the topics by old url?
Thank you!

[BirdOPrey5]

If you are using any of the default URLs available in either VB3 or VB4 then yes your old URLs will automatically redirect to the new VB5 style URLs.

If you used to use VBSEO URLs then they will not redirect to the new VB5 URLs. If you didn't purchase the VBSEO product then there is nothing to worry about.

[caves.ru]

I am using standard "Mod Rewrite Friendly URLs" and rules:

# Forum
RewriteRule ^threads/.* showthread.php [QSA]
RewriteRule ^forums/.* forumdisplay.php [QSA]
RewriteRule ^members/.* member.php [QSA]
RewriteRule ^blogs/.* blog.php [QSA]
ReWriteRule ^entries/.* entry.php [QSA]

RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d

RewriteRule ^.*$ - [NC,L]

# MVC
RewriteRule ^(?.*?)(?:/|$))(.*|$)$ $1.php?r=$2 [QSA]

[BirdOPrey5]

To be honest I don't know if it will work or not, you could try copying those rules and putting them above all the other rewrite rules in the VB5 .htaccess file on a test site, it may work, it may not. Officially I can only confirm Basic, Standard, and Advanced Friendly URLs will redirect to VB5 URLs.