"A" records don't have multiple IP addresses. What you have are multiple "A" records, each with their own IP address.
Treat the results of the Get-DnsServerResourceRecord the same way you'd treat anything that returned (possibly) multiple objects:
Get-DnsServerResourceRecord -Name BOGUS -ZoneName ATHOME.COM -RRType A |
ForEach-Object{
$NewRecord = $_.Clone()
$NewRecord.TimeToLive = New-TimeSpan -hours 2
Set-DnsServerResourceRecord -NewInputObject $NewRecord -OldInputObject $_ -ZoneName athome.com
}
The SOA record sets the minimum TTL for resource records. Whether that applies to new records only, or to all records in a zone, or to just all records in a zone that have a TTL lower than the SOA calls for I don't remember.