Can I get H1B stamping before the start date on I-797?

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I currently reside in USA and on H1B status. I have an I797 that is valid until Aug 30 2023, although visa stamp in my passport is expired. I have a H4 visa stamp from a few years ago. Never got H1B stamping done after I did COS from H4 to H1B. My H1B extension is also approved from Aug 30 2023 to Aug 29 2026. I was planning to visit US consulate in early Aug to get my H1B stamping done. If I get the stamping done in early Aug, I will be re-entering USA couple of weeks before the start of the validity date of my new I-797.

I am being told by my attorney to that I should wait until after new I-797 start date (Aug 30 2023) to go for stamping. He says that if I go for stamping and re-enter before the start date mentioned in my new I-797, my new H1B will not be valid. I don't understand how H1B can be invalidated if I get stamping before the I-797 start date and re-enter the country. Can anyone confirm if this is true?

TIA.