Install/Upgrade a different release of JUNOS software

 file copy http://10.1.1.254/jinstalls/jinstall-8.3R2.8-domestic-signed.tgz /var/tmp/
 request system software add validate unlink /var/tmp/jinstall-8.3R2.8-domestic-signed.tgz

root@R1> request system software add validate unlink /var/tmp/jinstall-8.3R2.8-domestic-signed.tgz
Checking compatibility with configuration
Initializing...
Using jbase-8.2R4.5
Verified manifest signed by PackageProduction_8_2_0
Using /var/tmp/jinstall-8.3R2.8-domestic-signed.tgz
Verified jinstall-8.3R2.8-domestic.tgz signed by PackageProduction_8_3_0
Using /mfs/validate/tmp/jinstall-signed/jinstall-8.3R2.8-domestic.tgz
Using /mfs/validate/tmp/jinstall/jbundle-8.3R2.8-domestic.tgz
Checking jbundle requirements on /
Using /mfs/validate/tmp/jbundle/jbase-8.3R2.8.tgz
Verified manifest signed by PackageProduction_8_3_0
Using /mfs/validate/tmp/jbundle/jkernel-8.3R2.8.tgz
Verified manifest signed by PackageProduction_8_3_0
Using /mfs/validate/tmp/jbundle/jcrypto-8.3R2.8.tgz
Verified manifest signed by PackageProduction_8_3_0
Using /mfs/validate/tmp/jbundle/jpfe-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-M10-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-M120-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-M160-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-M320-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-M40-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-M7i-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-T-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-X960-8.3R2.8.tgz
Verified SHA1 checksum of jpfe-common-8.3R2.8.tgz
WARNING: hw.product.model='unknown' using jpfe-M40
Using /mfs/validate/tmp/jbundle/jdocs-8.3R2.8.tgz
Verified manifest signed by PackageProduction_8_3_0
Using /mfs/validate/tmp/jbundle/jroute-8.3R2.8.tgz
Verified manifest signed by PackageProduction_8_3_0
Hardware Database regeneration succeeded
Validating against /config/juniper.conf.gz
mgd: commit complete
Validation succeeded
Installing package '/var/tmp/jinstall-8.3R2.8-domestic-signed.tgz' ...
Verified jinstall-8.3R2.8-domestic.tgz signed by PackageProduction_8_3_0
Adding jinstall...
Verified manifest signed by PackageProduction_8_3_0

WARNING:     This package will load JUNOS 8.3R2.8 software.
WARNING:     It will save JUNOS configuration files, and SSH keys
WARNING:     (if configured), but erase all other files and information
WARNING:     stored on this machine.  It will attempt to preserve dumps
WARNING:     and log files, but this can not be guaranteed.  This is the
WARNING:     pre-installation stage and all the software is loaded when
WARNING:     you reboot the system.

Saving the config files ...
NOTICE: uncommitted changes have been saved in /var/db/config/juniper.conf.pre-install
Installing the bootstrap installer ...

WARNING:     A REBOOT IS REQUIRED TO LOAD THIS SOFTWARE CORRECTLY. Use the
WARNING:     'request system reboot' command when software installation is
WARNING:     complete. To abort the installation, do not reboot your system,
WARNING:     instead use the 'request system software delete jinstall'
WARNING:     command as soon as this operation completes.

Saving package file in /var/sw/pkg/jinstall-8.3R2.8-domestic-signed.tgz ...
Saving state for rollback ...
Removing /var/tmp/jinstall-8.3R2.8-domestic-signed.tgz

The validate option checks that the new software is compatible with your current router configuration file. When you are updating to a different release of the JUNOS software, the validation check is performed automatically. The unlink option removes the software package from the router as soon as possible to make more room on the hard disk for the installation to complete.

Setting isupgrade=jboot-8.3R2.8.tgz

All together now...Somebody please give me some more memory!
List of Memory hogs ....
        cp size 49
        newfs size 20407

Junos requires a minimum of 198MB RAM to run…after an upgrade. After that 48-64 Ram is enough.