Because a reliable online tool does not exist, the safest, most effective way to convert your file is by using the native "Save As" feature within 3ds Max. If you do not have the newer version installed, you can download a free 30-day trial of the latest 3ds Max from Autodesk to perform this task.
Qingmo Tools offers a more straightforward, web-based conversion service that's simple and fast to use. Convert 3ds Max File To Older Version Online