changing scale

In general, you need to ratio the two scales (ratio is a fraction).  So it would be either 72/350, or 350/72.  Now, you need to decide which fraction.  You use logic for that.  Are you scaling it up- making it bigger?  Then 350/72 is obviously larger than one, so that is what you want.  Or are you scaling it down- making it smaller?  Then 72/350 is what you want, since it is less than one.

Summarizing- take the two scales as a ratio.  Scaling up requires a number bigger than one, while scaling down requires a ratio less than one.

A few years ago there was a scratch build of the I-400.

yes I agree on the size but maybe  it could be done in sections, more room for error.

more thought on this I must do

From what I heard, the I-400s surrendered at the end of the war, were studied by the Navy, and sunk, partly to keep the Russians from examining them.  There's at least one at Bikini, and the rest are deep in the Pacific.

A shame really, one would have made a heck of a museum display.

I don't know enough about 3-d printing to comment. My guess is that such a thing might be do-able - IF there's a 3-d printer big enough to handle such a job.

What I was thinking was to laser the hull, parts, from the kit, put it into a pdf then construct them in a 3d printer. To off the wall huh ?

i presume you will use the 1/350 sub as a template.

1/350 scale 13.7245"x1.3474"x.7885"

1/72 scale 66.71666"x6.55"x3.833"

scale you need to use is 4.86113641

I guess I don't quite understand the question. Are you talking about scratch-building a model of the I-400 in 1/72 scale? That would be a huge project. The finished model would be five and a half feet long.

The first thing you'd need would be a good, highly detailed set of plans. ("Scaling up" the 1/350 kit wouldn't be practical.) I don't know much about plans for Japanese warship plans; maybe somebody else in the Forum can tell you a good source.

Then you'd have to pick some materials. Maybe wood; maybe you could start with a piece of PVC pipe. In either case, the materials themselves would be expensive.

Then you'd have to scratch build virtually every other part. The only help the manufacturers would be able to give you would be a 1/72 seaplane.

Lindberg sold - briefly - a couple of 1/72 Japanese sub kits. They got pretty bad reviews in the modeling press, and, as I remember we're only on the market for a few months before the company went out of business. You might be able to get hold of one on E-bay or someplace.

Did I misread the question?