I stand at 6'4" inseam 35", early life was custom fitted to a Trek 820 felt like I was riding high really high.

Fast forwarding now 25 years later is it really necessary ,could I get the same fit by upgrading seat stem and handlebar stem to compensate for a smaller frame?

Or will the smaller frame work against me geometrically ? If so how critical?

On a bike that you ride around the block once a month a fit is irrelevant. On a bike that you ride 15-25 miles a day, and 50+ on weekends, it's critical for avoiding unnecessary pain, strain, and even injury.

Factors that contribute to the need for a new fitting: Age, fitness level, bike geometry, your level of demand for efficiency and/or comfort, distance ridden, frequency of rides, riding style. When I ride a lot, the difference of a half centimeter of cleat placement or saddle position can make a difference in how my knees tolerate the repetitive stress. And my fit changes a little as my fitness ebbs and flows. But I could jump on my kids' bikes and tool up and down the street for five minutes and fit wouldn't matter at all.

There are extremes...

What size of frame are you riding?

I'm guessing you could fit a bike somewhere between 60cm and 66cm, depending on a few factors like how low you want your bars.

To a large extent, yes, you can adjust the bike to feel comfortable by raising the bars, longer stem, shorter stem, stem angle, seatpost up/down, longer/shorter cranks, seat forward or back. And, the new threadless stems make it somewhat easier to adjust the stem if you have a new bike with an uncut steer tube. Likewise, larger diameter, longer seatposts are readily available.

I would think there is a limit to the adjustability though. So, you won't do well on a 50cm frame, unless it is designed to be particularly long.

If you're asking, yes you'll need to be fitted.

Is adjusting your seat, steering & mirrors necessary when you drive? I know, bad example, but you get the point! Have it fitted to your body. It'll cost you less than sessions at the physiotherapist in a few years and you'll perform better during your rides.

One thing that you won't be able to adjust when using a smaller frame is the stack & the reach. Both are important. There's a few threads here on that.

For some, yes. For me, I've ridden a lot of miles on many bikes and have never had a fit. I'd often buy bikes because I knew I could flip them if I didn't like them, but I've never had a fit before and have ridden quite a few centuries.... As far as setting up a bike, I do the rudimentary adjustments like saddle height and stems if necessary, but I learned all I need to know about that off of YouTube. Sometimes if something doesn't feel right, I'd tinker with it until it stopped bothering me. But a paid fit? Never. I'm a cheapo.

Not true, but proper stack and reach on a too small bike might cost as much as a fitting. I have commuted many miles and years in perfect fit on bikes with reaches too short for my very long arms. I have local framebuilders make me custom stems. (2) 180s, a 175, and a 155 so far. Much, much cheaper than the custom frame to get the same fit. Every once in a while a stock frame shows up that works with a 130-140. Big fan of Nitto 130 Pearl stems - 140 in the conventional measurement.

Ben

In other words, you knew what you were doing and fit yourself.

Worth $50 IMO

Paid fit is worth it if you can afford it, is comprehensive enough and is convenient for you.

For myself I searched around on the internet and bike forums to get an idea. I only adjust the saddle height, aft/front position, these make a lot of difference to riding comfort on long rides, my longest distance is 40 miles and daily rides of 20 miles.

I do agree for short hops you don't need a bike fit, but if you are doing serious about biking long distances, then do go for a paid bike fit its probably worth it. Just check out the reviews of the place you do the bike fit. If possible ask if they could suggest an appropriate saddle for you (if the one you have is unsuitable). Bike fits should be in-depth and comprehensive otherwise there is no point.

I had a paid fit at one point, but it was because I was having pain and could not figure out the cause myself after trying all the basic stuff mentioned online. If the pain you are in > than your need for the $, do it. A repetitive injury will end up costing you more down the road in time off the bike and/or PT. There's a difference between niggling little discomforts (which often go away with additional exercise) and actual pain from repeating a movement with a bad fit.
Try to figure it out yourself if you can. If you can't, then a fit may be necessary.