Rulings
Q1: "My opponent used this card's When Digivolving effect to prevent my Digimon with 6000 DP or less from attacking or blocking. If that Digimon's DP increased to 7000 or more by an effect afterwards, can it attack or block?"
No, it can't. A Digimon targeted by this effect will be affected until the timing specified in the effect, even if its DP increases to 7000 or more after the effect activates.
Q2: If I use this card's When Digivolving effect on an opponent's Digimon with 8000 DP and its DP is reduced by -2000 so that it becomes 6000 in a situation where I have a blue or yellow Tamer in play or I have a card with the Armor Form trait in my trash, can I choose that Digimon as the target for the effect that prevents 1 Digimon from attacking or blocking?
Yes, you can.
Data Sources
Lv.4 GoldVeedramon EX4-027 R Armor Form | Vaccine | Mythical Dragon <Armor Purge> (When this Digimon would be deleted, you may trash the top card of this Digimon to prevent that deletion.) [When Digivolving] 1 of your opponent's Digimon gets -2000 DP for the turn. If you have a blue or yellow Tamer in play, or if you have a card with the [Armor Form] trait in your trash, 1 of your opponent's Digimon with 6000 DP or less can't attack or block until the end of your opponent's turn.
Lv.4 GoldVeedramon EX4-027 R Armor Form | Vaccine | Mythical Dragon Digivolve: 2 from [Veemon] <Armor Purge> (When this Digimon would be deleted, you may trash the top card of this Digimon to prevent that deletion.) [When Digivolving] 1 of your opponent's Digimon gets -2000 DP for the turn. If you have a blue or yellow Tamer in play, or if you have a card with the [Armor Form] trait in your trash, 1 of your opponent's Digimon with 6000 DP or less can't attack or block until the end of your opponent's turn.
Lv.4 GoldVeedramon EX4-027 R
Armor Form | Vaccine | Mythical Dragon
{{DigivolveFromNames|2|Veemon}}
<Armor Purge>
[When Digivolving] 1 of your opponent's Digimon gets -2000 DP for the turn. If you have a {{Colour|blue}} or {{Colour|yellow}} Tamer in play, or if you have a card with the [Armor Form] trait in your trash, 1 of your opponent's Digimon with 6000 DP or less can't attack or block until the end of your opponent's turn.Lv.4 ゴールドブイドラモン EX4-027 R アーマー体 | ワクチン種 | 幻竜型 ≪アーマー解除≫(このデジモンが消滅するとき、このデジモンに重ねられているカードを上から1枚破棄することで、消滅しない) 【進化時】このターンの間、相手のデジモン1体をDP-2000。青か黄の自分のテイマーがいるか、自分のトラッシュに特徴に「アーマー体」を持つカードがあるなら、次の相手のターン終了まで、DP6000以下の相手のデジモン1体はアタックとブロックができない。