A few remarks on your code:

• the .Replace() and .Contains() string methods work case-sensitive, so .Replace("demo","demo997182625") won't find and replace "Demo". To have it work case-insensitively, use the -replace operator instead.
• updated files can be reprocessed by Get-ChildItem, unless you have that part finish completely first. The easiest way to do that is by enclosing it between brackets
• I would only save the file if there was something updated (i.e. the new value was found after -replace ), otherwise leave it be
• Get-ChildItem returns both FileInfo and DirectoryInfo objects. Since you are interested in changing files only, append the -File switch
• best use the FullName property of the found file on the Get- and Set-Content cmdlets instead of the whole FileInfo object

$found = $notfound = 0
# surround the Get-ChildItem line with brackets, so it will finish before iterating on the result
# otherwise, it could reprocess files that were allready updated
foreach ($file in (Get-ChildItem -Path 'C:\' -Recurse -Filter 'demo.txt' -File -ErrorAction SilentlyContinue)) {
    # -replace uses regex, so surround the search string with '\b' boundary markers in order to do a whole-word search
    $content = (Get-Content -Path $file.FullName -Raw -Force) -replace '\bdemo\b', 'demo997182625'
    # test if the content now has the new value (again, use '\b' boundary markers)
    if ($content -match '\bdemo997182625\b') { 
        # save the updated file
        $content | Set-Content -Path $file.FullName -Force
        $found++ 
    } 
    else {$notfound++}
}            
Write-Host "Changed: $found"
Write-Host "Not Changed: $notfound"

P.S. If your search string contains characters that in regex have special meaning (see table below), you need to escape these with a backslash when using regex operators -replace and -match .

Special Characters in Regex