Generating random conversions

I would use \ifcase instead:

\documentclass{article}
\usepackage[first=1,last=7,counter=grooth]{lcg}

\newcommand\randFloat{{%
    \reinitrand[first=0, last=100, counter=var]
    \rand\arabic{var}.\rand\arabic{var}}}

\rand\gdef\OldRand{\thegrooth}

\newcommand\randGrooth{%
    \rand\ifnum\thegrooth=\OldRand \rand \fi
    \ifcase\thegrooth mm  % 0
      \or mm              % 1
      \or cm              % 2
      \or dm              % 3
      \or m               % 4
      \or dam             % 5
      \or hm              % 6
      \or km              % 7
    \fi
    \gdef\OldRand{\thegrooth}%
}

\begin{document}

\begin{tabular}{ll}
44.36\,mm =& \hspace*{10 ex} m\\
56.28\,dm=& \hspace*{10 ex} km
\end{tabular}

\bigskip
\begin{tabular}{ll}
\randFloat\,\randGrooth =& \hspace*{10 ex} \randGrooth\\
\randFloat\,\randGrooth =& \hspace*{10 ex} \randGrooth  
\end{tabular}

\bigskip
\begin{tabular}{ll}
\randFloat\,\randGrooth =& \hspace*{10 ex} \randGrooth\\
\randFloat\,\randGrooth =& \hspace*{10 ex} \randGrooth  
\end{tabular}
\end{document}

This solution uses pdftex primitives, so it needs pdflatex to run. The units are typeset with siunitx package, they are organized into an array by means of \csname.

\documentclass{article}
\usepackage{siunitx}
\sisetup{round-mode = places,round-precision = 2}
%
\pdfsetrandomseed12345 %
%
\newcommand{\randFloat}{\pdfuniformdeviate100\expandafter.\pdfuniformdeviate100 }
%
\expandafter\def\csname0\endcsname{\si{\milli\metre}}
\expandafter\def\csname1\endcsname{\si{\centi\metre}}
\expandafter\def\csname2\endcsname{\si{\deci\metre}}
\expandafter\def\csname3\endcsname{\si{\metre}}
\expandafter\def\csname4\endcsname{\si{\deca\metre}}
\expandafter\def\csname5\endcsname{\si{\hecto\metre}}
\expandafter\def\csname6\endcsname{\si{\kilo\metre}}
%
\newcounter{ucount}
\newcommand{\incUcount}{%
\addtocounter{ucount}{\pdfuniformdeviate6 }%
\ifnum\arabic{ucount}>6 \addtocounter{ucount}{-7}\fi %
\expandafter\csname\arabic{ucount}\endcsname}

\newcommand{\randGrooth}{%
\incUcount\stepcounter{ucount} &= \hspace*{6 em}\incUcount\setcounter{ucount}{0}%
}

\begin{document}

\begin{table}
\centering
\begin{tabular}{Sll}
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth \\ 
\randFloat & \randGrooth    
\end{tabular}

\end{table}
\end{document}