I tried the behavior with MC11.1.73. I installed it on top of MC11.0. The PC has also MC10 installed.
MC 11.1 has no problems with restoring an MC10 library backup file into a newly created empty library. Almost everything was restored nicely including the static playlists and many of my smartlists. However, the date & time based smartlists were empty, like "Imported his week". I think the time system changed in 11.1 at some stage, though my similar 11.0 smartlists work fine in 11.1 and on the other hand MC11.0 accepted the same smartlists from MC10.
However, the smartlists are a minor problem. Bigger problem is that MC10 cannot play anything after installing MC11.1.73. It hangs immediately. Sometimes it crushes the OS too and Windows shows a blue screen of death. When it does not crush the OS, Windows shows an error message about this file: visstudio.dll.
Media Center Registered 10.0.173 -- C:\Program Files\J River\Media Center\
Microsoft Windows XP Workstation 5.1 Service Pack 1 (Build 2600)
Intel Pentium 4 2523 MHz MMX / Memory: Total - 521 MB, Free - 323 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1400 / Shell32.dll: 6.00.2800.1233 (xpsp2.030604-1804) / wnaspi32.dll: 4.60 (1021) , ASPI for Win32 (95/NT) DLL, Copyright © 1989-1999 Adaptec, Inc. / Aspi32.sys: 4.60 (1021)
Ripping / Drive J: Mode:Normal Type:Auto Speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: No / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: Yes / Play sound after ripping: No
Burning / Drive J: QSI DVD/CDRW SBW-161 Addr: 1:0:0 Speed:16 MaxSpeed:16 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: Yes
Use playback settings: No / Normalization: None
Media Center Registered 11.1.73 -- C:\Program Files\MC\
Microsoft Windows XP Workstation 5.1 Service Pack 1 (Build 2600)
Intel Pentium 4 2512 MHz MMX / Memory: Total - 521 MB, Free - 307 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82.2800 / Shlwapi.dll: 6.0.2800 / Shell32.dll: 6.0.2800 / wnaspi32.dll: 4.60 (1021) , ASPI for Win32 (95/NT) DLL, Copyright © 1989-1999 Adaptec, Inc. / Aspi32.sys: 4.60 (1021)
Ripping / Drive J: Mode:Normal Type:Auto Speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: No / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: Yes / Play sound after ripping: No
Burning / Drive J: QSI DVD/CDRW SBW-161 Addr: 1:0:0 Speed:16 MaxSpeed:16 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: Yes
Use playback settings: No /