Yes, the mIRC installer is designed to preserve your existing settings, scripts, and so on. However, please note:
1) As with all new versions of an application, when you upgrade, it is possible that your existing addons, scripts, etc. may need to be updated. This applies to all applications, eg. when new versions of Firefox/Chrome are released, sometimes any addons you use with them also need to be updated.
2) Some mIRC users have reported losing their settings after an update. The most common reason for this is anti-virus software not recognizing the new version of mIRC and wrongly deleting various mIRC-related files after the update.
To be safe, you should make a backup of your mIRC folder before updating, just in case something goes wrong. You can find your mIRC folder, with all of your settings and scripts, by typing the following command into the mIRC status window and pressing the enter key:
This will open up a file explorer window at your mIRC settings folder location that you can copy as a backup, in case you need to restore it later.