There are a few ways and it depends on how the AEP is setup. The most efficient is to use the ScaleComp.js script that comes with AE.
You should take note that if your SD is 4:3 then you'll end up with either a letter-boxed video or you will need to pan and scan.
Another method is to place your SD footage into a HD precomp and scale to fit comp. Then use this comp to replace your HD placeholders. Once you've reached the 'final' HD omp, nest it into an SD comp and enable the Collapse Transformation check-/box. Take note that the end-result may look awkward as the HD assets will be squashed
Sell your AEPs with broadcastGEMs' DVD series of templates. Click here for more
Import the desired SD footage into the project and use the Replace Footage command. Depending on how the Place Holders' Scale property was set, this SD footage will look either fine (scale=25% or God-awful (100%).
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA