pdftex with miniltx not working as expected

I have a four-year-old Windows 10 machine, with a four-year-old installation of MikTex 2.9, and a brand-new Windows 10 machine, with a brand-new installation of MikTex 2.9.

On both machines, I have the following file, called test.tex:

defgraph#1#2#3{
     includegraphics[height=#3]{#1.pdf}}
newcountpdfoutput pdfoutput=1

input c:/dos/belfry/graphixx
input c:/dos/belfry/eplain

centerline{graph{chinese}{3in}{3in}}

bye

(I do not use LaTeX; the above illustrates my standard workaround for displaying .pdf files in Plain (pdf)TeX output.

Both machines have identical files /dos/belfry/graphixx.tex and /dos/belfry/eplain.tex. The first line in graphixx.tex is
"input c:/dos/belfry/miniltx". Both machines have identical files /dos/belfry/miniltx.tex. Both machines have identical files chinese.pdf.

When I type "pdftex test" on the old machine, I get what I want: A pdf displaying the contents of the file "chinese.pdf". By contrast, when I type "pdftex test" on the new machine, I get the following message:

! Undefined control sequence.
@startdef ->expandafter edef @commandname

@newcommand ...expandafter @startdef @paramdef
                                                  {#1}
<to be read again>
                   [
l.25 providecommandDeclareRelease[
                                    3]{}

I would really really like this file (and my hundreds of other archived .tex files) to compile under the new pdftex. What do I need to do?

Remarks:

1. Clearly the software has changed; the file pdftex.exe is 50K on the old machine and over a meg on the new one. I’d be very happy if I could just find an installer that would install the old version on my new machine, making it identical to the old one.

2. Failing that, I’d like to force the new installation to work. There is presumably a clue in the fact that on the old machine, 9 files are loaded after miniltx, and on the new machine, only 7 files are loaded. The two missing files are both in tex/generic/oberdiek (under the main MikTeX directory) and are called infwarerr.sty and ltxcmds.sty . Copying those files from the old machine to the appropriate directories on the new machine does not help.