I have a Synology DS218+ with two 4TB drives configured as Synology Hybrid RAID (SHR). Both drives are healthy; all is well. NAS is running DSM 7.1.1.
I have two new 8TB drives to replace the existing (lower-capacity) disks. I’m nervous.
I’ve read the DSM Manual, but I’m hoping someone here can reassure me.
It’s been over a year since I tried this before. I had to abort, but here I am again rushing in like a herd of turtles.
I appreciate any advice.
First, backup your data and also your synology configuration.
I’ve replaced drives but that was on a 4 bay. So I wasn’t as concerned. The good thing is that since you’re on a 2 bay then each drive should essentially be a mirror of the other. Another good thing is that you’ll still have both original drives and can recover that way.
Just replace drive and let it rebuild. Run diagnostics. Then replace second drive and let it rebuild. That should be it.
I am in a similar situation (718+ here with 2x 4 TB). Eventually I will have to replace those drives in order to increase the storage later this year.
I agree with @r2d2 :
Backup your data and configuration. (I am doing that once a week locally and the important data daily to the Synology Cloud.)
Follow Synology’s instructions (To replace a drive (when there are no unused drives or empty drive slots)) from the website you have linked to.
I am knocking on wood for both of us (I am also not looking forward to this procedure, but we just should move on, I guess). 
Please keep us posted how everything went.
Based on those instructions, I do not think that you can replace a drive. They state that you must have at least one unused drive in your Synology; otherwise the Replace button will not be shown.
As you have a 2-bay unit, with SHR enabled, I would assume you do not have a spare drive available.
Because of this, there is no way (that I know of) to replace a drive without having your SHR array go into a degraded state (eg no data redundancy is available) when you replace a drive. The purpose of replacing is to allow you to replace a drive without entering into a degraded array state - which basically means you must have an unused drive for this purpose.
As far as I understand things, you will have to remove one drive from your array, putting it into a degraded state. Then you swap the physical drive, and add the new (larger) drive to your array, and give it time to rebuild. Once the rebuild is completed, then you do the same process for the second drive.
I agree that a backup of any important data should always be made before embarking on this process.
I am doing the same thing this week. My current 4 bay unit with 3 x 8TB and 1 x 6 TB is nearly full, and I have 2x 14 TB arriving (in theory) tomorrow. Absent an unused drive, I will also have to remove and add a drive, leaving my array in a temporarily degraded state. No way around that that I know of.
If someone knows better, I would appreciate that input!
I think what you described is correct. I have a DS918+ 4 Bay drive. I initially have 2x 8T and 2x4T HD, and I have to swap to 4x8T. Like you said, I have to swap one out at a time and during that process, the SHR will go into degraded state with 3-drive while rebuilding the newer and larger HD. The process took a day or may be 2. Then I let the system rest for a while before finishing up the last swap
I think it is an affordable risk
There is no reason, to be nervous about that.
SHR means, you have only 4TB of usable storage, and therefore the second drive is just a mirror of the other one.
Just remove one of them (I would leave the one with less hours, or the better health), replace this by one of the new drives, and let the SHR rebuild.
If this has been completed, continue with the other one.
If thinks really get wrong, you could but the first drive back in, and you have everything back like before the switch.