private ulong testMsgId = 0;

[Command("test1")]
public async Task Test1Async(CommandContext ctx)
{
    var  msg1 = await ctx.RespondAsync("test message");
    testMsgId = msg1.Id;

    msg1 = await ctx.Channel.GetMessageAsync(testMsgId);
    await ctx.RespondAsync($"A: {msg1.Author.Username} at {msg1.Timestamp}: \"{msg1.Content}\"");
}

[Command("test2")]
public async Task Test2Async(CommandContext ctx)
{
    if (testMsgId == 0) { return; }

    var msg1 = await ctx.Channel.GetMessageAsync(testMsgId);
    await ctx.RespondAsync($"A: {msg1.Author.Username} at {msg1.Timestamp}: \"{msg1.Content}\"");

    if (msg1.Content != null)
    {
        await msg1.ModifyAsync(
            msg1.Content,
            new DiscordEmbedBuilder { Description = "test", Timestamp = DateTimeOffset.Now }
            );
    }

    msg1 = await ctx.Channel.GetMessageAsync(testMsgId);
    await ctx.RespondAsync($"B: {msg1.Author.Username} at {msg1.Timestamp}: \"{msg1.Content}\"");
}