4r50w transmission delayed engagement in 1st both upshift and downshift.
once in 2nd, it cycles though the other gears fine both up and down. Longer delay after sitting for a long period.... like overnight.
transmission has over 300k miles with no maintenance. Fluid is brownish pink with no metal shavings in it. No weird noises or clunking in any gear. No hard shifts except when I have to give it a gas pedal nudge to engage 1st gear.
where should I start?
also, no codes found or o/d flashing.

 

If they only made transmissions that lasted forever ! With no maintenance, I would say you received your monies worth. There are parts internally that have gone beyond their period of service. It's decision time, replace or rebuild or keep a towing company's phone number handy in the car.
Under the circumstances , changing fluid and filter now probably would not make any difference and could make things worse.