You certainly can change the report and let only assets be listed which have a purchase date older than seven years. Save it under a new name, because the modified report won't really be related to manufacturer warranty any more. Please find an example below:
Select Top 1000000 tblAssetCustom.AssetID,
tblAssets.AssetName,
tblAssets.IPAddress,
tsysAssetTypes.AssetTypename,
tblAssetCustom.Model,
tblAssetCustom.Manufacturer,
tblAssetCustom.Serialnumber,
tblAssetCustom.PurchaseDate As [Purchase Date],
tblAssetCustom.Warrantydate As [Warranty Expiration],
tsysAssetTypes.AssetTypeIcon10 As icon
From tblAssetCustom
Inner Join tblAssets On tblAssetCustom.AssetID = tblAssets.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Where tblAssetCustom.PurchaseDate < GetDate() - (7 * 365) And
tblAssetCustom.State = 1
Order By [Warranty Expiration] Desc