Like different SATA connectors (SATA1, SATA2 & SATA3) the SAS connectors are compatible. So a SAS1 HDD can be connect to a SAS2 port and SAS2 HDD can be connect to a SAS1 port. This is a connector compatibility, without the speed improvements.
Additional thing to remember is SAS connectors, because of the way the connectors are constructed, SAS connectors are compatible with SATA HDDs. It is part of the SAS specification. a SAS connector can take a SATA drive, but a SATA cable cannot take a SAS drive. A SAS connector can connect to SATA HDD, but a SATA connector cannot connector to a SAS HDD.
Refer to the picture shown below for a pictorial comparison between the 2 connectors types.
SAS/SATA Compatibility Simplifies Storage by Design