Premiere can handle all kinds of resolutions in one sequence.
SD will suffer qualityloss in a HD timeline.
You can try AE (latest version)which can upscale the SD footage now with good results.
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Adobe Certified Expert Premiere Pro CC
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You cannot select standard or widescreen.
Premiere will capture the footage how it was recorded to tape.
If the footage is anamorfic widescreen it will be captured in 4;3 but Premiere will interprete the footage as widescreen.
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Adobe Certified Expert Premiere Pro CC
Adobe Community Professional