Since you've updated BIOS to a supporting one it should go OK...but nothing is guaranteed in this world. Maybe death. And taxes.
When you're ready for the exchange be sure to reset CMOS after installing the new processor, that's the number one thing people fail to do and then have stability problems. Also check MSConfig...Boot tab...Advanced Options and make sure the check box beside Number of Processors is cleared. It sometimes get set to the number of the previous CPU's threads.
Heat is a concern as Ryzen is thermally sensitive...keeping the CPU cooler means it boosts higher and longer. So if you haven't yet, consider getting a really good cooler to reap it's fullest benefits. Also something to consider is good case ventilation with proper fans in front and top/rear to push cool air in and hot air out.
There is one last BIOS update that ASUS offered that I have not installed (don't fix what isn't broke) but the update for 2nd & 3rd gen Ryzen's is installed. It may not be a bad idea to go ahead & install that last update.
If I recall correctly, the 1st gen Ryzen CPU / MB combo were fussy about accepting RAM. What I was assured by online store to be fine had to be exchanged for RAM approved on the ASUS list.
The Ryzen 1700 is 8 core, 3900X is 12 so probably some changes in the BIOS Processor box.
Will I need any of my passwords after resetting CMOS? And, will Microsoft need to be called because a detected change of hardware? I don't want to be surprised by not having that & other info.