Change vertical alignment mid-frame

Question

I have three important points that I want to make in succession on one frame, appearing slide by slide. The second point needs some clarification though, so I want to interject with a slide that contains the point, and also a note about it. The three main points are center aligned, but this note might be fairly lengthy, so I want to top align the slide that contains the note (as with the [t] option). Ideally I would like this all to happen on one page. That is, the page number doesn't increase through any of these slides.

A minimum working example:

\documentclass{beamer}

\usetheme{Ilmenau}

\begin{document}

\begin{frame}
    \begin{onlyenv}<-2,4->
        \begin{block}{Block One}
            This is my first block.
        \end{block}
    \end{onlyenv}
    \begin{onlyenv}<2->
        \begin{block}{Block Two}
            This is my second block.
        \end{block}
    \end{onlyenv}
    \begin{onlyenv}<3>
        \begin{block}{Note}
            This is a really long note about block two that takes several lines.
            When it appears on slide 3 the entire frame should be top aligned,
            rather than centered as on slides 1, 2, and 4.
        \end{block}
    \end{onlyenv}
    \begin{onlyenv}<4->
        \begin{block}{Block Three}
            This is my third block.
        \end{block}
    \end{onlyenv}
\end{frame}

\end{document}

To be perfectly explicit, I want Block One to appear centered on its own in slide 1. In slide 2, Block One and Block Two should appear centered. Slide 3 should contain Block Two and Note top aligned, and then slide 4 should contain Block One, Block Two, and Block Three, centered once again.

I have tried creating two more frames representing slides 3 and 4, but then there's a lot of duplicated code, and I also can't seem to reconcile the page count. I have also tried with \againframe for just the third slide as well, but I have the same issues.

Answer 1

You could use a [t]op aligned frame and insert a \vfill at the top on all overlays with should be centred:

\documentclass{beamer}

\usetheme{Ilmenau}

\begin{document}

\begin{frame}[t]
    \only<1,2,4>{\vfill}
    \begin{onlyenv}<-2,4->
        \begin{block}{Block One}
            This is my first block.
        \end{block}
    \end{onlyenv}
    \begin{onlyenv}<2->
        \begin{block}{Block Two}
            This is my second block.
        \end{block}
    \end{onlyenv}
    \begin{onlyenv}<3>
        \begin{block}{Note}
            This is a really long note about block two that takes several lines.
            When it appears on slide 3 the entire frame should be top aligned,
            rather than centered as on slides 1, 2, and 4.
        \end{block}
    \end{onlyenv}
    \begin{onlyenv}<4->
        \begin{block}{Block Three}
            This is my third block.
        \end{block}
    \end{onlyenv}
\end{frame}

\end{document}