There are some pros and cons for using SBD convert function.
But regardless of its intended original use, any user on Steem can use this function, which will convert 1 SBD into roughly 1$ worth of STEEM, after 3.5 days, at the median STEEM price over that period.
Although the Convert function was removed from steemit.com, being deemed for advanced users only, you can find it in SteemWorld by @steemchiller on the Balances page, the "Convert SBD..." button, like in the image below:
If you click the button, a popup window will show where you can actually set the amount of SBD you want to convert into STEEM, along with the warning of when you should use it:
Like the warning says, this function should only be used when SBD is traded at below $1, because you would receive roughly $1 worth of STEEM after the conversion ends. However, there is no guarantee the price doesn't go up during this period, but the usage of a median should offer a good protection against sudden pumps of SBD price. If you use the function when SBD trades above $1, especially way above $1, it means you will suffer a loss, so be careful!
To go on with the conversion, you will need to enter your active key after clicking OK on the popup window above.
If you'd rather use SteemConnect for the convert operation, @timcliff describes the entire procedure in detail here:
/sbd/@timcliff/how-to-convert-sbd-into-steem-using-steemconnect
I know there is also another tool especially written for converting SBD to STEEM. I don't remember details about it, but maybe you want to search for it.
If you want to use the Convert SBD function, there are ways as you can see, both simple (like in SteemWorld) and more complex. Personally, I don't need it re-enabled in steemit.com.
The tricky part is understanding it takes time. Your SBD will simply seem to vanish from your balance and nothing is added to your STEEM balance or elsewhere, at least not for 3.5 days.
The only record you have of the conversion starting lies on the blockchain. Here's how it looks like on Steemd.com:
We will see how that shows up on the blockchain after 3.5 days, when the conversion finishes.