After reconfiguring the Site sync settings on the source environment, the list with items pending for sync are removed. Is there a way to restore them?
If the Site sync target url gets changed will the pending items for sync be preserved, or will it treat it as a new server and show all content as needing to be synced?
After canging the site sync target server on DEV for site sync to PY and all of the pending sync updates are lost.

The situation occurs in the following scenario:
1. Sitefinity site sync is configured to sync towards destination.com
2. Content and pages get created for a week and there are 100 items awaiting site sync
3. A need to change the url of th destination server url from destination.com to new.com
4. This update is performed by deleting the site sync destination url destination.com and a new url to new.com gets added.
At this point the 100 items pending for sync are no longer available. The pending items for new.com are 0.

The previously pending items for sync are lost. This is because they were bound to the server url which got updated.

When the Site sync settings are reconfigured (the target server is removed and a new one is added), all pending items are removed due to change in the serverId attribute in the SiteSyncConfig.config
Example:

<add serverAddress="http://<site domain>" userName="<username>" password="<password>" serverId="b16e410c-83a3-4781-8256-3f25fd45124d" />

The new destination url for site sync has new serverId.

An update needs to be performed in SiteSyncConfig.config
In order to restore all pending items for sync, the previous serverId has to be obtained and its value has to replace the currently used serverId.

<add serverAddress="http://<site domain>" userName="<username>" password="<password>" serverId="b16e410c-83a3-4781-8256-3f25fd45124d" />

1. In case a backup of SiteSyncConfig.config is present check the value of serverId attribute and copy it to the currently used SiteSyncConfig.config. After this restart the site and all items pending for sync from the previous serverId will be made available for sync again.

2. In case no backup of SiteSyncConfig.config is present, check the sf_site_sync_log_entry table with the following SQL query:

SELECT COUNT(*), serverId
FROM sf_site_sync_log_entry
GROUP BY serverId

3. Use the serverId value and place it in SiteSyncConfig.config and update it. After this restart Sitefinity in order to apply the update. All pending items for sync(if there are of course items pending for sync) will show up after the old serverId is used.

There should be multiple serverId values in the table. The old serverId which carries items pending for sync which were lost with the change of the destination server URL can be obtained from here. One approach to identify the correct serverId is to check the "timestamp" and "item_id" columns and identify the correct entry based on the information for an item that was pending for syncing.